There is a rival theory to dark matter and that’s MoND (Modified Newtonian Dynamics). As its name implies, MoND proposes that Newton’s law of gravity must be changed in order to explain the missing mass in our universe.
Proposed by Mordehai Milgrom in 1983, MoND only works when the gravity of an object is large. Yet, and even by Milgrom’s admission, MoND may not hold when gravitational accelerations are small. In comparison, dark matter seems to account for the missing matter in the universe without having to modify any laws of physics at all.
Despite this, however, some scientists support MoND. Even Vera Rubin, who found the first direct evidence for dark matter, leans towards this idea.
Image Credit: NASA/ESA