Question

What is dark matter?

Answer 1

"Dark Matter" is a missing actor in a Gravitational Play. Maybe we are observing an Electromagnetic Play, an Homopolar Motor! 
Maybe, nature uses Electromagnetism not Gravity to control the galactic stars motions. 
The energy E=mcV is the solution for the "Missing Matter " problem,. mcV is the energy providing the speed of stars in the galaxy.. The velocity is V = E/mc. This derives from the Electric Model of the galaxy, a homopolar Motor, , where the galaxy rotates as current comes in on the edge and Jets out of the center. Charged stars rotating create a B magnetic field perpendicular to the galaxy. In effect there is a mass spectrometer for stars. 

The equation is the Lorentz force F=qVxB = mv2/r giving velocity v= qrB/m = qruI/rm =quI/m=qzI/mc=E/mc! 

The velocity is independent of the radius r and dependent on the current I and the ratio q/m of the stars. 


The vector energy E=mcV is the "Dark Matter" of the Missing Matter in the galaxies and the "Dark Energy" of Gravitational Theory

Answer 2

It is defined as the (in some cosmological theories) non-luminous material which is postulated to exist in space and which could take either of two forms: weakly interacting particles ( cold dark matter ) or high-energy randomly moving particles created soon after the Big Bang ( hot dark matter ).