Convergence

Convergence happened when Earth-1 and Earth-2 started to fuse. Then the threat of the antimatter combination came the antimatter from both dimensions were going to merge and that would send a wave antimatter into the matter dimension which would destroy both Earth in the process. Fortunately the heroes of both dimensions were able to stop it. they were able the turn both Earth's back to normal. In the aftermath of that the leader of The Multiverse Guardians created Earth-1.5