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I have a man-in-the-street question that was probably "predetermined". If everything around us is co-interacting particles whose source is some infinite small point that started their expansion into nothing due to the Big Bang, how come those particles have any interactions? They should become further with every second, just as grenade fragments that can only meet each other after the explosion only if there are some external preexisting obstacles on their way that cause their bounce.

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  • $\begingroup$ Although this comment's too vague to serve as an answer, my impression's that the Big Bang concept has, except within the "local universes" of a "multiverse", been dealt a death blow by the discovery of new galaxies thru the James Webb Space Telescope, which are covered in many YouTube videos at various levels of sophistication. One possible exception, that's applicable to a "single universe" by application of the "Weyl Curvature Hypothesis", is Roger Penrose's much-discussed "Conformal Cyclic Cosmology", that has endlessly (and beginning-lessly) iterations (AKA "past- & future-eternality). $\endgroup$
    – Edouard
    Jun 17, 2023 at 4:39

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The main reason that some of the initially separating particles came back together is their mutual gravity. Within some parts of the expanding universe, the density of matter was high enough that gravity stopped the expansion. These regions collapsed under their self-gravity to (eventually) form galaxies. Everything around you formed inside a galaxy.

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At the beginning, those particles were indeed interacting fiercely with one another but the energy they carried was sufficient to propel them apart.

At the same time, gravity was trying to bring them back together, but it was too weak then to do the trick.

Note that as the original particles expanded freely, they were cooling off, and at some point their initial energy was diluted enough by expansion that gravity could take (feeble) hold and begin very gently urging them into clumps.

The weakness of gravity meant that this initial clumping required something like a hundred million years to produce goodly-sized clumps. These were destined to later form black holes, galaxies, and stars.

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