citing Will Percival, director of the Waterloo Centre for Astrophysics:
According to the history revealed in the eBOSS map, the expansion of the universe began to accelerate six billion years ago and has continued to get faster ever since.“That’s just really whacky because if you just have a standard theory that gravity is acting on matter, gravity is an attractive force it pulls things together and it would in general tend to decelerate the universe,” says Percival. “Which means there’s physics out there that we don’t know about.”*While there are theories, “none of them are really compelling.” According to Percival, the simplest mathematical explanation would be taking Einstein’s cosmological constant equation and change one sign in it to change the universe from static to make it accelerate. Mathematically, it’s a simple thing to do but when it comes to the physics it’s much more difficult to explain.
*a link – see a note on notes and links