Dark matter, because it gravitationally attracts, it tends to encourage the growth of structure, OK.

But because gravitationally attracting matter is embedded in this space, it tends to slow down the expansion of the space, OK.

So the evidence that we have that a quarter of the universe is dark matter this gravitationally attracting stuff is that galaxies, the speed with which stars orbiting galaxies is much too large it must be embedded in dark matter.