I've been reading about dark matter. I understood that its existence is inferred by the discrepancy between the gravity exerted by a galaxy (total mass) and the movement of the stars that compose it.
In other words, the outer stars would not behave as they do if the total mass of the system was not much higher than can be seen.
I wondered if there are other possible explanations for this phenomenon instead of dark matter.
Answer
The evidence for dark matter is extremely compelling provided that Einstein's theory of relativity is assumed. To get out of dark matter means dropping that assumption (or that we've SERIOUSLY botched some observations). Some people study alternate theories of gravity. The two alternate theories (or classes of theories? keywords?) I hear most about are f(R) gravity and MOND: MOdified Newtonian Dynamics.
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