Observations of how stars move around the galaxy reveal that the Milky Way's mass is about a trillion times that of the sun, far more mass than is accounted for by the combined masses of the stars visible to us.

Because we can directly measure only a fraction of the mass in our galaxy, most of the mass is believed to be in dark matter contained in the galactic corona.

But just giving something a name doesn't explain it. We really don't know what this stuff is. Dark matter is just a catch-all phrase for all the galactic contents we can't see directly.