Question: What is dark matter?
Answer: We don’t know.
Well, that was easy, see you next week!
…No, okay, I know that wasn’t actually a satisfying answer, even though it’s the truth. Even if we don’t know what dark matter is, I can explain why it’s important. It’s important — necessary might even be the right word — because what we see in our observations of the universe requires an explanation. We can see a lot of matter out there in the universe, but various different calculations all tell us the same thing: there isn’t enough (visible) matter!
So for one reason or another, there is a lot of mass out there in the universe that we just can’t find. Hence the term “dark matter”. It’s matter that we’re sure must exist, in order to explain everything we see, but because we can’t see it or interact with it, it’s a total mystery to us. Scientists have theorised various exotic new types of particle that don’t interact with normal (“baryonic”) matter, in order to explain what dark matter is, but so far there’s no proof of any of the theories.
Searching around just today has told me that scientists aren’t universally agreed on dark matter. Go figure! Other theories suggest that there’s something different wrong with our current understanding, and maybe the theory of general relativity should be altered. Again, there’s not much evidence one way or another.
Right now, you might even be reading in places like Gizmodo that there’s a new theory that dark matter consists of particles older than the Big Bang. More importantly, this theory is testable, and we can start looking for evidence to disprove it from 2022, with the Euclid space telescope. So… watch this space!
In the end, this is still a really unsatisfying answer, but what’s exciting about it is that it means there’s a lot of interesting science left to be done on the subject!