This is an unsolved problem. If someone tells you they know the answer then they are probably mistaken. https://en.wikipedia.org/wiki/Loschmidt%27s_paradox htt

This is an unsolved problem. If someone tells you they know the answer then they are probably mistaken. https://en.wikipedia.org/wiki/Loschmidt%27s_paradox https://en.wikipedia.org/wiki/Loschmidt%27s_paradox The only convincing-seeming resolution I've heard is that our universe started in an exceptionally low-entropy state at the Big Bang.