I was recently in a talk about Dark Matter and Dark Energy, and I completely yield to the astounding evidence that Dark Matter exists because gravitational lensing had been observed. However, Dark Energy is inferred from the expansion of the universe, but I am not totally convinced by this solution. But to begin, we should start with an explanation of why we think there is even a problem with gravity in the universe.
In short, there is insufficient gravity for galaxies to be held together, so we need Dark Matter. The problem necessitating Dark Energy is even worse. All of gravity is attractive, so the realization that the universe is accelerating in its expansion means that Dark Energy is unequivocally there.
For alternative hypotheses, each realm of physics has their own interpretation. Particle Physicists love to posit extra particles and dimensions. People working on GR suspect that the laws of gravity are just plain wrong in some way, even though Einstein's GR has already won over the gigantic list of alternatives to GR posited over the years.
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