Death's End question

It is, IMO, perfectly elaborated in the fan-made sequel to this trilogy, Three Body X.
It is said (rumored?) that Ken is working on its translation.

Find in the spoiler below Baoshu's answer to your question, at the very end of Three Body X:
(view spoiler)[
The universe got "rebooted", only after an epic battle fought between two factions, one of which was leveraged by Yun Tianming and another character from Death's End (guess who?).
However, there are not so many "little universes" as Universe 647. In fact, Sophon in Universe 647 was no longer representing Trisolarans; she'd be leveraging the other faction instead. Nor was Universe 647 created by Trisolarans - otherwise there would be so many little universes that the reset becomes statistically impossible.
Sophon's persuading Cheng Xin to leave behind the messaging bottle was one of her attempts to prevent the universe from resetting, as that was the ultimate goal of her faction.
With no more spoiler to reveal (not enough yet?), the universe did reset after a long story. But the lack of those 5 kilometers (of the messaging bottle) has caused extremely tiny, but still distinguishable differences.
The Alpha Centauri still consists of three stars, but two of them are closer, in a stable binary system, with the third relatively farther away - exactly as we observe in the real world. The Trisolarans are satisfied with their living on the planet and are not imposed with immediate need to emigrate and invade.
On the Earth, most protagonists in the trilogy live similar but not exactly the same lives. Da Shi didn't make it through a narrow escape in a combat in the Sino-Vietnam war; Cheng Xin was forced to be returned to the Welfare facilities instead of being kept by her stepmother; Ye Wenjie's research was spoiled by her sister; Mao made a different budget decision between the Red Coast project and the Niangziguan Thermal Power Plant (where Cixin Liu has served for decades), favoring the latter instead. The last but not the least: Yun Tianming is perplexed with his memories from the "last" universe; finally he decides to put them down in the form of sci-fi.
You may get it right - Tianming is now with a different name. You know his name and the book's name now, don't you?
(hide spoiler)]

Cixin Liu said that the universe has been reset finally, the 5 kg did not cause the fail of resetting.

Sean wrote: "Assuming the universe did reset after all the dimensions collapsed, and then the messages in the bottles were released, what happened? Is our universe doomed to repeat the dark forest, and continue..."

Yes. Any people with "selfless" genes who'd be inclined to coexist and assist the other survivors regardless of race will sacrifice themselves while those with "selfish" genes will hide out in their pocket universes. After the reset, the selfish people will emerge and start messing with the new universe just as they messed with the old universe.

Ultimately, even if other humans survive in other pocket universes, they're doomed anyway. We have to assume that some 10 dimensional people figured out the pocket universe trick and used it to escape the transition to 9 dimensions (and that this trend continued onward through the other transitions) so if humans wait for the universe to step down to 3 dimensions (or even 4 given that they seem to be able to function in 4) the universe will *already* be in a Dark Forest state by the time they come out of their cocoons.

cheng xing finally find the rest of human race，then they decided to seach a place as old erath to live.
suddenly,chenxin received Message from another civilization."Do not answer! Do not answer!! Do not answer!!!There are tens of millions of stars in your direction. As long as you do not answer, this world will not be able to ascertain the source of your transmission.
But if you do answer, the source will be located right away. Your planet will be invaded. Your world will be conquered!
at last ,answer still received。
and then a huge univers space fleet go into deep silver river
they will about arrive that beautiful planet 400 hundred years later...