system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Dark Matter

What is it? I understand that current believe is that most matter in the universe is dark matter and only about 4.6% of matter consists of atoms(quick wikipedia lookup). However nowhere do I see explained what dark matter actually is. I see theories about it's existence but still, what is dark matter?
gpt
We can calculate how gravity will affect an apple, or the Moon, or entire solar system and equations work, orbits are predictable, ect ect. When we look at galaxies and clusters and very big objects and try to calculate how they should behave we get wrong results (stuff was spinning at wrong speeds). So either laws of gravity is wrong, or not fully understood, or there is additional something in the universe that affects it. We can't see it, can't catch it, but it must be there, because gravity is affected by it. 


Best we can do right now is strap a placeholder name to it, throw a couple of equations explaining it and call it a day. 



TL;DR It's either some sort of exotic matter we can't see, or quality of space we do not know of yet, or our inability of comprehending gravity.