How can dark matter be expanding the universe?
You are confusing dark matter and dark energy. Dark matter interacts gravitationally with all the visible matter in the universe. We can’t directly see Dark Matter but we see its effects. What you mean is the Dark Energy that is thought to be responsible for the accelerated expansion of the universe. We actually know nothing at all about its nature, just that it makes around 70% of the universe (with around 25% Dark Matter and the rest is ‘normal’ visible matter and neutrinos and such).
Dark matter is needed because if the only gravitationally acting thing in a galaxy were visible matter, the galaxy would fly apart: stars are revolving too fast around galaxy centres to be hold on their orbits through the effect of visible matter only. The same is true for galaxy clusters: they would not be bound structures, were it only for the visible matter. The deficit there is still worse than in galaxies. Cold normal matter in the form of aborted stars or brown dwarfs has been looked for to explain that but not found. One should have seen it occulting the stars behind, one hasn’t seen it. Hence the idea of dark matter made of a different stuff, that some people have tried to explain away through a modification of Newton’s and Einstein’s gravity. But DM is also traced through the gravitational lensing which it causes to the light from shining objects. And this cannot be explained by these modifications of gravity laws. So there is strong observational evidence for DM now. Dark ene
