Could the Dark Matter be Some Sort of Strange Matter?   If the dark matter is not dead stars and such (which it could be, but will be practically impossible to prove, if it is), or brown dwarfs (which it cannot be, as they aren't numerous enough), or neutrinos (which just don't seem to weigh enough), then what could it be?   One possibility that has been raised, is that the dark matter is something, like neutrinos, that are scattered throughout all of space, but are very difficult to detect -- in fact, something that is so difficult to detect, and so different from any normal form of matter, that no one even knows that particles of this sort exist at all. This material is called Strange Dark Matter.   Of course, if we don't even know that the particles which make up strange dark matter exist, or anything at all about them, or how to observe them, why should we believe that they even exist, let alone make up most of the mass of the Galaxy, and perhaps, of the Universe? The answer is, perhaps we shouldn't do so. But wouldn't it be exciting if such material did exist, and Someday Soon, we could figure out what it is, and how to observe it, and found, all of a sudden, that this was indeed the answer? Of course it would! So, for quite some time now, this has been the preferred answer to what the dark matter might be.   Now, as to what this stuff would be, there are various sorts of answers, according to who is proposing them. Some theories of physics propose the existence of various types of particles which have never been observed, so that their properties are somewhat uncertain (although there are predictions as to some of their properties, as a result of the theory which predicts their existence). Among such particles are axions, Higgs bosons, vector mesons, gravitons, magnetic monopoles, and Weakly Interacting Massive Particles (WIMPs). Some of these particles are believed to definitely exist, but in unknown numbers, and with unknown masses, while others may or may not exist, outside the minds of the people who imagined them. So far, however, none of them have been shown to exist in even small numbers, let alone the staggering numbers that they would need to exist in, in order to explain dark matter. However, for various reasons, for the last 30 or so years, these particles, or something similar in characteristic, but otherwise different, from these, have been thought to make up most of the mass of the Universe. However, to explain why that is the case, we need to discuss dark matter in the Universe (to be written and posted at some future date, when the answers are more nearly educated guesses than mere guesses). Final Thoughts on Dark Matter in Galaxies   When you have read everything there is to read about dark matter in galaxies and in the Universe, you should keep certain things in mind. Regardless of what the nature of dark matter in galaxies is, regardless of what it is made of, or why it is so difficult for us to observe, we know that it is there -- we just don't know what it is. It may be some strange form of ordinary matter, such as dead stars which are impossible or nearly impossible to observe, or some still stranger form of "strange" matter, which is like nothing that we have ever observed at all. But regardless of what the answer to the question is, the masses of the galaxies are at least ten times greater than the masses of their visible stars, and so there is something there, something that we don't know about, something which we would dearly love to know about, which grips our imagination and fuels the efforts to find it.   On the other hand, when you have read about dark matter in the Universe, you will find that if it exists, it cannot be dead stars or any other kind of "normal" matter, no matter what form that matter might be in. If it exists, it must be made of some kind of "strange" matter -- perhaps not any kind of strange matter so far proposed, but definitely something which is not in any way like any kind of matter ever observed. However, that kind of dark matter, as you will read later, may not exist. In fact, there is absolutely no doubt that if it does exist, it is much rarer than was once believed. At one time, it was thought that between 90 and 95% of the so-called "critical mass" of the Universe consisted of strange dark matter, whereas now, we know that if it exists at all, no more than 20 to 30% of the "critical mass" of the Universe consists of such material (though that is still several times the amount of "normal" visible matter, which is only thought to be about 4 to 5% of the "critical mass". And, of course, since we have no observations that say what it is, or whether it even exists at all, perhaps it exists only in our imaginations.   So why have I even bothered to mention strange dark matter? The reason is that for many years, it was thought that strange dark matter made up most of the mass of the Universe (as discussed in the yet-to-be-written "dark matter in the Universe", and as discussed in your text), and if so, that it ought to also make up most of the dark matter inside galaxies. So when talking about dark matter inside our galaxy, which is known to exist, you will read that it must be mostly strange dark matter. That is not true. It is possible that strange dark matter exists, and if so, then it may make up a large part of the dark matter inside galaxies. But it is also possible that strange dark matter does not exist, and that all of the dark matter inside galaxies is just some form of ordinary matter in a very strange form. We just don't know. All we know is that there is something that makes up 90% of the mass of the galaxies, about which we know almost nothing except that it exists.