Dark matter pre-dates its detection by billions of years: hence its name. It is dark precisely in the sense that it does not emit or reflect light—it doesn’t interact with electromagnetic radiation at all. It is detectable only by means of remote gravitational effects. It is nowhere to be seen, though it constitutes most of the universe’s matter. Dark matter is to be distinguished from invisible matter—the kind that is too small to see (even with a microscope) or too distant to transmit light to planet Earth. Invisible matter is ordinary matter consisting of protons and neutrons (“baryonic matter”) and it interacts with light, whereas dark matter is another kind of matter entirely. A black hole is not made of dark matter, though it is necessarily invisible, because it is ordinary matter (though highly condensed) and does interact with light. Both types of matter contrast with visible matter, which is the kind that emits light signals that reach our eyes or telescopes. We could say that the universe contains three sorts of matter: visible, invisible, and anti-visible. There is the visible, the contingently invisible, and the necessarily invisible. It is in the nature of dark matter not to be visible, an aspect of its physical make-up, unlike the matter of galaxies too far away to be detectable by means of emitted light. Dark matter cannot be known by electromagnetic means; it actively prohibits visual revelation. It is intrinsically un-seeable.
Yet it is matter in the ordinary sense (though not in the ordinary way). It has mass, a particulate structure (though of unidentified particles), extension in space, gravitational effects, and impenetrability. It really is matter, despite being undetectable via light. Thus it lacks an epistemic property possessed by both visible and invisible matter: they can be known in a certain way and it cannot. We might say that it is a contingent property of matter that it is knowable via light—some matter is not knowable in this way.  It is true to say that dark matter is invisible, like invisible non-dark matter, but it is strongly invisible—invisible by nature not circumstance. For humans it belongs at the lowest rung of accessibility, though it is no less real for that—or less material. Clearly it would be wrong to define matter in terms of visibility: that would be to conflate epistemology with ontology. The right thing to say is that some matter is positively opposed to exposure: it wears a cloak of invisibility (or has internalized such a cloak). There is nothing magical or “unempirical” about dark matter; it is just matter that refuses to engage with our best means of knowing about the physical world. And why should the physical world feel the need to oblige our epistemic faculties? It is just an accident that a proportion of it interacts with light in the way that our eyes can exploit—it might all have been dark. What we have discovered is that certain astronomical anomalies require the postulation of this kind of invisible matter; thus we need additional matter over and above the matter previously recognized. The material universe is more extensive than we thought, and some parts of it are less accessible than others. The universe contains a hidden subterranean region; not all of it can have a light shone on it. Indeed, humans might never have detected dark matter (as it was not detected for the whole of human history until the twentieth century); it might have lurked in darkness forever, cloaked in the obscurity that is its wont. Its telltale signs are few and slender (and still rejected by some theorists); yet its existence is robust. We just happen to be perceptually cut off from it.
But this is not an essay about astronomy: it is an essay about the mind. For what I wish to suggest is that a comparable tripartite division may obtain with respect to the mind: that there is a conscious mind, an unconscious mind, and a dark mind. The dark mind is aptly characterized as an anti-conscious mind: it actively resists conscious accessibility. It is in its nature not to be available to conscious scrutiny. It is unconscious by necessity. As it were, it refuses to interact with conscious light. It is not even the kind of thing that could be conscious. Yet it is fully and completely mental. Let me state the view with maximum ferocity: there exists a part of the mind that has all the usual characteristics of the mental and yet it is necessarily not conscious. By the usual characteristics I mean: intentionality, phenomenology, combinatorial fecundity, and functionality. It might be an alien type of mind, not sharing its specific features with our conscious mind, yet it is indisputably a type of mind (not just brain circuits or some sort of computational-functional system). It is like dark matter in being both a type of mind in good standing and yet cut off from awareness: it is a type of mind that is opposed to consciousness. The dark mind might be composed of elements hitherto unidentified, operating by different principles from those of the conscious mind: but it is a type of mind nonetheless. It is just that consciousness is not its medium, its lifeblood, its sine qua non. We are never conscious of it and never can be.
Thus I distinguish the anti-conscious from the unconscious, as astronomers distinguish the anti-visible (dark) from the merely invisible. We may speak of the “ordinary” unconscious and populate it according to predilection: presumably it contains dormant memories, banks of cognitive machinery, and maybe a soupcon of Freud. These all belong in the invisible category as opposed to the dark (or strongly invisible): they could become conscious, or they are the kind of thing that might be. I don’t want to get caught up in the metaphysics of the unconscious here, or in specific issues about what kinds of unconscious exist, so let me just say that memories are the kind of thing that belong to the unconscious in the ordinary sense: they can be conscious and are not designed to elude consciousness. In fact, they pop into consciousness all the time—there is nothing dark about memories, even if some may be beyond recall (cf. those distant lightless galaxies). In the case of Freudian theory, the matter is not so straightforward, because of the mechanism of repression; but it was supposed that repression could be reversed and the repressed materials brought to consciousness. Also the Freudian unconscious mind seems remarkably like a conscious mind that has slipped coyly behind a curtain. This mind is not anti-conscious in that it is entirely removed from consciousness, existing separately from it in a realm of its own, not even interfacing with consciousness. What I am talking about is a mind that exists completely below the radar and has probably existed in this way for millions of years (ever since Mind came along in the course of evolution). It pays no heed to consciousness and consciousness pays no heed to it (but see below). Its darkness is total. It revels in its inaccessibility.
The first thing I want to say about such a mind is that it is conceptually possible; it contains no contradiction. There could be a mind that is completely cut off from consciousness. Just as there can be an unconscious mind, so there can be a necessarily unconscious mind—unconscious by nature or design, not by contingent circumstance. The second thing I want to say is that such a mind is not incompatible with what we know of psychology, both the commonsense kind and the scientific kind. The third thing is that it wouldn’t be all that surprising if such a mind exists: for brains are complex and ancient organs with many a crevice and chamber, and it may well be that long ago they hatched mental attributes that never ascended to the level of consciousness. Maybe the human brain is a hotbed of minds that hover below the radar (“junk minds”, like junk DNA): there could be all sorts of mental reality thrown up by the brain that never see the light of day. Isn’t it strange that some parts of the brain produce mind and some do not? Well, maybe more parts do than we realize, given the limited reach of consciousness. What if our brain houses remnants of mind descended from the brains of our ancestors, from fish to ape? The brain is a wondrous organ still largely unplumbed and it may cater to minds both light and dark.
Now someone may object that there is a difference between dark mind and dark matter, namely that there is no evidence for dark mind, but there is for dark matter. Admittedly the evidence for the existence of dark matter is remote and disputable (if now generally accepted), but at least there is some evidence for it: dark matter makes sense of a number of astronomical puzzles that nothing else seems to. It can seem like an extravagant hypothesis in view of the odd anomalies it is wheeled in to explain, what with the vast expansion of matter postulated in the universe, but at least the hypothesis has a firm basis in observable reality—what can dark mind point to of comparable evidential value? In order to answer this question we need to ask if there are any areas of perplexity in psychology comparable to those in astronomy before dark matter was introduced: where does psychology, commonsense and other, draw a theoretical blank? And would dark mind do anything to resolve such puzzles? Notice that, even if there were no evidence, that would not refute the hypothesis—it would just put it beyond the reach of empirical test. And it would not be surprising if dark mind had no observational support, given the nature of the hypothesis. We might simply never know whether we harbor a dark mind (or minds). Such a mind might exist undetectably.
But I think there is one promising area to look at—or look at again: dreams. It is hard to discuss this subject without feeling the looming figure of Freud, but it may be that though he had many things wrong about dreams he had some things right. Maybe he had the general architecture right but not the specific furniture. I have no intention of venturing into the murky territory of dream interpretation; I will just say dogmatically that Freud was right to postulate a parallel mental system responsible (at least in part) for dream content. To put it in astronomical terms, dreams are an observational anomaly that calls for additional mental machinery. This is because they cannot be explained by means of standard psychological processes. Again, I will not defend this assertion, though I think it is evidently correct; my point is that if it is true then we have a reason to postulate a dark mind. Not just a mind that happens to be unconscious, or which resembles our conscious mind in all but its consciousness, but a mind that is resolutely unconscious and decidedly peculiar—fundamentally alien. It is irrational, associative, unrealistic, anxiety-ridden, bizarre, nonsensical, hysterical, infantile, grotesque, and probably insane. It bears little resemblance to anything of which we are conscious. We can infer some of its properties from dream content, but we cannot be directly aware of it. Just as we know that dark matter consists of particles but can’t specify what they are, so we know that dark mind consists of mental elements of some kind (and principles of assembling them) without being able specify what they are. Dark particles might be a bit like visible particles, but crucially different in other respects (no interaction with light); and the constituents of dark mind might be a bit like the constituents of the conscious mind, but also crucially different (no interaction with introspective awareness). The mind that gives rise to dreams is not a mind like any of which we are conscious. It exists inside us but it never enters consciousness in its primal form. Dreams are symptoms of this mind but not the thing itself.
But now the question is how the dark mind can intrude upon the conscious dreaming mind: how dark can it be if it so intrudes? It can be as dark as dark matter is: for intruding upon consciousness is not the same thing as being an object of consciousness. Dark mind could affect the course of dreams without it being possible for us to be conscious of this mind. Dark mind acts as the cause of an observable phenomenon (dreams) without disclosing itself to observation: it operates invisibly, like dark matter. It can even be anti-observable and still have observable effects—it just lacks observational effects (i.e. it doesn’t cause observations of itself). Dark matter causes gravitational effects that can be seen, but it cannot be seen—it cannot produce observations of itself by means of light transmission. Evidently in dreams something alien intrudes upon our conscious life—something not present in waking consciousness—and it is possible that this is the (or a) dark mind buried deep within the brain. It never surfaces as a mental reality in its own right, but it produces perturbations in other more accessible parts of the mind. To use a physical metaphor: it bends the light of consciousness without itself being conscious. Or again: there is a leakage from the dark mind into the luminous mind. That could be so even if dark mind is vigorously anti-conscious.
Once we bring in dreams other sorts of mental pathology might be considered. Are mental illnesses in general caused by the dark mind not being sufficiently sealed off from consciousness and bringing disorder to it? It’s not the brain as such that figures in the etiology of psychopathology, but something distinctively mental, and possibly disruptive. Of course, this is totally speculative, but it illustrates how a dark layer of psychological reality might have empirically observable effects that betray its existence. The general form of the inference is familiar from psychoanalysis: what I am adding is the idea that the unconscious layer might be strongly unconscious (as well as alien)—that it might be intrinsically incapable of conscious access. The Freudian unconscious is made of once-conscious material that has been repressed, and is thought to be potentially accessible to consciousness; but the anti-conscious mind is conceived as inherently beyond the scope of consciousness—despite being genuinely mental. Just as dark matter can’t be seen in principle, so dark mind can’t in principle be introspected or otherwise made conscious: yet both may robustly exist and be capable of influencing the observable course of events.
If there were literally a light of consciousness, we could say that dark mind can’t interact with this light. There would then be two kinds of mind–the luminous mind and the non-luminous mind—and they would be differently composed. Their different kinds of composition would explain their different relations to luminosity—they would be composed of different “particles”. Maybe dark mind is made up of very primitive mental components forged in the early evolution of mentality, long since superseded by more sophisticated mental components; nevertheless it survives covertly in the “mental universe”, occasionally making its presence felt. (Might dark matter have been formed in the cosmos before luminous matter was formed and be more primitive than it is?) If it preceded consciousness, it would not be surprising if it were inaccessible to consciousness—if it shunned consciousness by its very nature. Still, it might impinge on consciousness, possibly distorting and troubling it, without being integrated with it. Dark matter can speed up visible matter because of its gravitational heft; maybe dark mind can “speed up” consciousness, or at least jumble it up. It can cause the conscious mind to go haywire. Who knows? Something does and we really don’t have any better explanation.
Is dark mind mysterious? No doubt it is: but so is dark matter (without being magical). We know very little about it, if anything. In particular, we don’t know why it’s dark—what causes it to be dark. We know it doesn’t interact with conscious awareness, save indirectly, but we don’t know why this is (we don’t know much about consciousness either). There seem to be strata of the mind that are more or less proximate to the conscious mind, with the anti-conscious mind at the lowest level and the unconscious mind one level up. The lowest level is moreunconscious than the other levels (and each level may contain different types of non-conscious mentality). Dark mind, like dark matter, is particularly elusive, given its limited interactions with other things; both are easy to miss, though they may be quite extensive. Here, as elsewhere, we must not let epistemology dictate ontology. Just because the dark mind is not accessible to consciousness and barely evident in daily life is not a reason to deny its existence: it might exist and it might play a significant role in human mental life (and perhaps that of other animals). I picture it as a seething cacophony of ill-coordinated elements, prone to conflation, horribly irrational, egocentric, vaguely reptilian, and clearly simian. The sexual is just one aspect of its makeup (pace Freud): it is also obsessed with locomotion and bodily functions (inter alia). Quite possibly it is prone to synaesthesia. But maybe that picture reflects only how the dark mind is refracted through the dream mind, not its intrinsic character. Maybe it is actually quite far removed from anything we can experience or easily conceive, possibly consisting of relics and fragments of bygone minds, both ontogenetic and phylogenetic. It might be dark in many ways.
 This is not to say that dark matter cannot be perceived in other ways: presumably it can be touched and heard, and maybe tasted and smelled. But light is the standard way we know about the properties of distant objects.