Mathematically many things, like parallel universes and time travel, are possible. Folding 8 pints of water into the volume of a single pint is, too. (A tesseract). But at a practical level, there's no evidence that they can work.
Dark matter was first proposed back in the 1920s, but only in the past few decades, well after Einstein's death, has it been researched seriously. Even so, many scientists are still trying to prove whether it exists or not, and what it could be.
Denigrating Einstein is hardly fair. He understood more about the universe than most of his contemporaries.