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This question is about how small the interstellar density is (less than 1 atom per cubic centimeter).

But although the interstellar density is very low, matter tends to extremely aggregate in certain clusters within and through the whole universe (we call them e.g. galaxies, solar systems and planets). Of course this is explained in traditional physics by gravitational attraction of matter on itself.

Are there other current theories - than gravitational attraction of matter - which explain the reason for the extremely clustered accumulation of matter in the universe?

Addendum:

If gravitational attraction of matter was the only reason for the existence of the universe, why isn't the universe just a big clumb of matter since the big-bang (which btw is proven due to the expanding universe)? Doesn't the expanding universe and its deduced big-bang prove that there must be certain invisible processes in the universe which we know nothing about whatsoever? And that these processes tend to organize matter - maybe even create matter in the first place?

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    $\begingroup$ Do you understand that there are only four known fundamental forces? Have you considered each one? $\endgroup$
    – G. Smith
    Oct 1 '20 at 22:56
  • $\begingroup$ Yeah, and none of these forces actually explain the clustering of matter in the universe, except gravitational attraction of matter of course...but there are reasons that speak against this force as well. I might elaborate more on that, wait a sec. $\endgroup$
    – Marcus
    Oct 1 '20 at 23:03
  • $\begingroup$ @downvoters: Please elaborate on why you downvoted. (I say you don't understand the underlying problem, because your downvote just proved that...) $\endgroup$
    – Marcus
    Oct 1 '20 at 23:13
  • $\begingroup$ Doesn't the expanding universe and its deduced big-bang prove that there must be certain invisible processes in the universe which we know nothing about whatsoever? Are you asking about what caused the Big Bang? Or are you suggesting that General Relativity can’t explain the expanding universe? $\endgroup$
    – G. Smith
    Oct 1 '20 at 23:57
  • $\begingroup$ I didn’t downvote, but you seem unaware of current theories of structure formation, and interested in promoting some kind of non-mainstream physics, perhaps some personal theory. Those are off-topic here. $\endgroup$
    – G. Smith
    Oct 1 '20 at 23:59
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Gravity acting on both matter and dark matter is the only candidate in mainstream physics for the force that determines the large scale structure of the universe (although the electromagnetic force may also influence smaller scale structures within individual galaxies).

The early universe was not completely smooth. Primordial fluctuations in density that formed the seeds for the first galaxies have been observed in the cosmic microwave background. The origin of these density fluctuations was quantum fluctuations magnified enormously by the brief period of cosmic inflation very shortly after the Big Bang.

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  • $\begingroup$ "Gravity acting on both matter and dark matter" - So you accept that there is some form of gravity that is not caused by matter indeed? And could this be considered the fifth fundamental force in the universe? (Which we currently assume to be caused by "dark matter", but this term is more showing current zeitgeist than the underlying cause of this gravity, because there is no proof that this invisible matter exists, this term was only created because we can't explain a lot of gravity in the universe without it). $\endgroup$
    – Marcus
    Oct 2 '20 at 10:55
  • $\begingroup$ @Marcus There is only one "form" of gravity. Ordinary matter and dark matter interact gravitationally in exactly the same way, and the force of gravity does not distinguish between them. Indeed, dark matter was detected precisely because it interacts gravitationally with ordinary matter. $\endgroup$
    – gandalf61
    Oct 2 '20 at 11:30
  • $\begingroup$ @gandalf61 I think I see his point... He's arguing that dark matter is really just a hypothesis to explain gravitational anomalies. It hasn't been detected directly. So his question is; if there is no dark matter, could there be another force? Or another term in the gravitation equation? $\endgroup$ Oct 2 '20 at 11:49
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    $\begingroup$ @OscarBravo Yes, the questioner seems to be going in that direction, in which case we are outside of mainstream physics and cosmology (or, at least, at their outer fringes) and all bets are off. It could be all down to invisible pink unicorns ... $\endgroup$
    – gandalf61
    Oct 2 '20 at 12:02
  • $\begingroup$ Well, the problem seems to be a two-headed demon: On one side, mainstream physics (let's call it that way) sets the axiom that gravity can only be caused by matter. On the other side, understanding the universe is only possible by the introduction of "dark matter". But if we'd remove the axiom, we would not need to introduce a hypothetic matter which no one has ever seen or measured. It would not only be much easier to introduce the fifth fundamental force named "non-material gravity",... $\endgroup$
    – Marcus
    Oct 2 '20 at 23:22

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