What we don’t know about dark matter:
Dark matter takes up about 84.5% of all mass in the universe, and we practically have no idea what it is. Dark matter doesn’t interact via electromagnetism, meaning that you can’t see it, feel it, or interact with it in almost any way possible. If you held a lump of it in your hand, it would just fall straight through without you ever noticing it was there to start with. So, if it’s almost perfectly invisible, how do we even know it exists at all?
When looking at a galaxy, you can estimate how much matter is in it by what you see through a telescope, and you can use this to predict how fast the galaxy should be spinning. However, there’s a problem. Galaxies always appear to be spinning much faster than they should be. In order to be spinning as fast as they are, galaxies need a lot more mass than what we’re seeing. Even when we account for things that are a lot harder to see, like planets, dust clouds, neutrinos, and black holes, the numbers just don’t add up. So, this leaves us with two options; either Einstein’s theory of gravitation is wrong, or there is a new, invisible type of matter filling up galaxies.
Since Einstein’s theories seem to be extremely robust under any other circumstance, we are left with the possibility of a new type of matter that can only interact through gravity. Although we can figure out how much dark matter is in the universe, and where it is mainly located, we are nearly clueless on the details. After all, you can’t just look at a clump of dark matter through a microscope.
Since it’s possible that dark matter could also interact via the weak nuclear force, there have been several super-sensitive detectors built to look for extremely rare dark matter interactions, but none have been able to find anything significant yet. If dark matter is a new particle, there’s a chance it could be created at the Large Hadron Collider, or we could at least see its effects on other particles, but the LHC hasn’t seen anything out of the ordinary yet either.
So, although we have a good idea of what dark matter is doing to our universe, we have almost no idea about what it actually is. Whenever we do finally figure out the true nature of dark matter, it will surely be the discovery of the century.