If we knew the answer to that question, then we wouldn’t call it “dark matter” anymore. 🙂
The thing that started the whole dark matter search was that someone noticed that galaxies are not rotating the way we thought they should. We have a mathematical description of gravity that tells us with incredible precision how objects such as stars, planets, moons, satellites move around each other under the influence of gravity. It all depends on the masses of the objects involved, and on their speed and direction of motion. If we know all this, we can predict how the motion will continue, for example which orbit a satellite will take.
Now, what happened a while ago was that people started looking at galaxies in the same way. We can see the stars in there, we can estimate how much mass the stars have, we correct for a few big dust clouds and so on, and hey, there you can predict how the stars in a galaxy move, especially how quickly they rotate around the centre of the galaxy.
Except that the result was completely wrong, and we found that the outer regions of galaxies rotate around the centre a lot faster than they should. There are two possible explanations for this. Either gravity behaves differently on very big distances such as the ones in galaxies. After all, our knowledge of gravity comes from much smaller objects, mostly within our own solar system. But so far most alternative theories of gravity haven’t explained the exact nature of the deviation from our expectation very well.
In comes option 2: you can *easily* explain the different motion if you assume there is a lot more mass in a galaxy than what we see. The additional matter must be invisible to us, otherwise we would have seen it. We see objects in faraway galaxies because they either send out light (such as stars) or reflect or dim star light (dust clouds). So this additional matter is invisible in the sense that it is dark. Dark matter!
Now, we don’t know what it is, mostly because most of the “simple” explanations just don’t fit the bill.
Could it be black holes? Nah, black holes actually make quite a fuzz if stuff is falling into them, so you should see the effects of matter falling into them.
Could it be burnt-out stars (“brown dwarfs”)? Nah, because they don’t all burn out at the same time, we should see many stars still glowing a bit, or still being warm enough to be visible in infra-red, but there is way too few of those.
Could it be gas clouds floating around between the stars in the galaxy? Nope, this gas should heat up enough over time to start glowing in infra-red, or there should be enough of it to block the view onto many of the stars, and that’s not the case either.
Could it be neutrinos? Neutrinos have a little bit of mass that we usually don’t take into account because it’s tiny, but there are *so many* neutrinos out there that it could have an effect on galaxies. And neutrinos don’t interact with light because they have no electric charge, meaning we could really not see them. But no, also neutrinos are not a suitable explanation. Because they have such a tiny mass, they aren’t really sticking to galaxies. They fly around almost freely, whereas the dark matter is clearly stuck within galaxies.
Yeah, and this is pretty much where we run out of “normal” candidates. The conclusion is that there must be additional particles that we haven’t been able to see in the Universe because they are dark, and that we haven’t been able to produce with our particle colliders yet, either because they are too heavy for us to make, or because they are only created so rarely that we missed them. There are many theories predicting suitable particle candidates, but until we’ve found at least one of them, we really won’t make much progress…
Nobody knows! That’s why it’s called dark matter. The main reason we know it exists is because of its gravitational effects on ‘normal’ matter, but apart from that we know very little. We have some clues though.
Firstly, dark matter needs to be made up of something that’s extremely heavy, otherwise there’s no way it would constitute 90% of the mass of the universe. It also needs to basically not interact with normal matter, or we would have detected it already.
With those two assumptions, there are 2 main dark matter candidates. One is called a WIMP (weakly interacting massive particle), a new type of fundamental particle which is similar to some of the more exotic particles we already know of. The other candidate is something called a sterile neutrino. The neutrinos we know of react very infrequently with matter, but are also extremely light, ruling them out as dark matter candidates. There do, however, exist theories which predict the existence of a new type of neutrino which is both extremely heavy and interacts with nothing *at all*.
There are some experiments being built in the next 10 years which might, if we’re lucky, solve the dark matter mystery once and for all. It’s one of the greatest unknowns in particle physics and an intense area of research.