While the dark matter hypothesis appears to be the most compatible with current cosmological models, there might not be dark matter. Two of the most popular alternatives to dark matter are the Modified Newtonian Dynamics (MOND) and some sort of electromagnetic coupling (Electric Universe Model).

MOND is simply adding an extra term in Newton's Gravity Equation, but the fit is not as good as originally appeared.

The Electric Universe Model holds a lot of promise and intrigue. Electromagnetic coupling between the sun and the planets is shown dramatically in the auroras found on Earth, Jupiter, Saturn and Uranus that are caused by electric Birkeland currents from the sun.

Well the unsolved mystery is stimulating developments in Theoretical and Experimental Physics. Have fun with that.