Computer simulations carried out by astronomers from the University of Groningen in collaboration with researchers from Germany, France and Sweden show that most of the (dark) matter beyond the Local Group of galaxies (which includes the Milky Way and the Andromeda galaxy) must be organized in an extended plane. Above and below this plane are large voids. The observed motions of nearby galaxies and the joint masses of the Milky Way and the Andromeda galaxy can only be properly explained with this "flat" mass distribution. The research, led by Ph.D. graduate Ewoud Wempe and Professor Amina Helmi, is published in Nature Astronomy.
Dark matter is like x and y in a quadratic equation in maths. We don’t know what it is but we know it is there. It effecting the equation such that you are getting a result.
We know there is something, it is effecting the gravity around it to result the apparent motion of the stuff around it. But we don’t know what it is or how it is working.