Year 22~~ rocks around, time travel gets invented, and naturally is also discovered by all of the governments and corporations which you Don't want time travelling.
So, "Chrono-terrorism" has a brief hike, with billions of dollars going into every different slice of the world sending agents back to carve out their own little paradise.
None of them come back. The initial idea is that they failed, got shot by the SS with the rest of the would-be saboteurs, but when the history books adamantly refuse to show Nazis having suddenly jumped to laser technology, they look elsewhere.
Eventually, some very well-paid men with pocket calculators deduce that the universe doesn't care. The would-be time travellers drop the 1939 edition of Nature on the desk of the head Japanese scientists, the whole Second World War takes a different turn, and a pocket universe is formed. However, it is so tiny, so insignificant and meaningless, that the universe just fixes it. Four billion dead insects on one little blue marble in one bag of marbles in the big box of bags of marbles. A paradox so paltry that the big Rule can just scrub out the errant speck of dust, and bring that universe back into the fold.
So, the hook of the story begins a few years after this revelation. A very intelligent, very ruthless cookie manages to make waves, cause a calamity large enough that the universe sits up and takes notice.
Somewhere, billions of years ago in the great cosmic uncertainty- a thing happens.
A detonation, an implosion, a trivial rewrite of physics, and a certain section of space disappears. It's only a few galaxies, and beyond the observable universe, inconsequentially distant from all human life. However, it's huge. A few bags missing is apparently large enough.
Oh, and kid Deepthroat is throttled in an alley somewhere.
So, wondering whether the story should focus on people trying to reverse the dystopia, or just have it as an overly-complicated excuse for "and then the aliens attacked".