Dimensions. Alternate dimensions? Multiple timelines? Many worlds?
I can see them in my head. Think of the "Bulk", the totality of timelines, the arc of the multiverse, as a city full of skyscrapers. Some people think of it like a tree. That's wrong. A tree fills in the empty space. It grows denser as it grows. The Bulk is defined by its empty space. Instead of a tree branching off forever, imagine rising up from the street-level of a city, and how the buildings would look as you went. You'd ascend past the tops of some of them; those that survived might stay the same, or their facades might narrow like the Empire State Building, but they certainly wouldn't get larger as you went up. The Bulk grows less dense, infinitely less dense, as it goes on. Most of the world is missing.
|Map of the universe - read from the bottom up, not top to bottom|
Similar timelines tend to converge — this is what people famously perceive as the Roediger Effect, when their memories clash with "recorded" """history""". Some presents, many pasts, one future. Scholars of the Bulk believe that each timeline (there's more than one, and travel between them is almost convenient) is miming one ur-timeline and one series of events (this is the Blue Stoplight Effect), and one day that Universal Goal will be achieved and the world will end (?).
To reiterate: the multiverse does not diverge into infinite timelines. The universe splits in two whenever a coin is flipped, sure, that's Quantum, but it doesn't matter. The two timelines will run together again shortly, like water flowing around an obstruction. The multiverse converges into one timeline, or none. When you go back in time you need to be careful not to step on a butterfly, or there will be swastikas flying over the White House when you get back to the present. But you're still impotent, and ineffective, and you can't change your own future, because universe used to care about butterflies, but it doesn't any more.
How many timelines are there? Not that many, really. High triple digits. The ones that have diverged from your own might seem pretty strange, but the really weird ones are the timelines that were never (yet) connected to yours. Universes where planets are empty spaces in infinities of rock. Universes where up is down. How does that make sense, you ask? I don't know. I don't come from such a place. I came from a timeline where — oop 15 minutes is up.