The Omniverse is all of existence in all present,future,or past timelines. Every single reality(dimension/universe/etc) exists in the omnivores. The Omniverse is catergorized of different groups of similar universes,called Multiverses.
"The Omniverse is like a giant potato tree of existence. Each branch is a multiverse and each potato is a universe...or something...Potatoes are fruit right?"-Lurk
Each Multiverse has millions of universes within them. The universes in a multiverse all originate from a common older universe,making them all similar in some form. Travel between universes has been done, an example being the Nightmare Dimension's involvement with Z-14 . Direct travel between Multiverses has not been disproven, but has never been attempted. Multiverse travel is incredibly difficult and is likely to be impossible.
Travel between universes can be done with extremly advanced technology. Such as warpgates,portals,etc. It can also be done by time traveling to alternate timelines. A more dangerous method of traveling to other universes is by using rifts. However,rifts are basically unstable rips in the frabic of reality,and can often be very unpredictable and dangerous.
it is unknown how the omniverse started. It is so infinite and vast that it might be impossible to study it fully.
One theory is that the omniverse could've existed forever,and that it has no beginning.
Unfortunately, on the other side of the timeline, scientists have confirmed that the Omniverse will end in a phenomenon called Omniversal collapse. It is not completely confirmed when it will end,and it is unknown what will happen after it ends.