We know that it exists only because its gravity pulls on the things we see. But it emits no known form of radiation, so we do not know what it is made of. The dark matter correlations provide a new handle on this slippery problem. For example, they disprove one theory about how most dwarfs form. Some astronomers have postulated that dark galaxies are produced when large galaxies collide.
Tides pull long tails of gas and stars out of the parent galaxies.
If small lumps in these tails are dense enough, gravity can hold them together. They can survive the collision and look like little galaxies afterward. The idea is almost certainly correct. And the vicinity of our Galaxy is a good place to look for tidal remnants, because we are currently interacting with two small neighbor galaxies, the Large and Small Magellanic Clouds. So tidally made dwarfs should have lower dark matter densities than their parent galaxies.