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Our universe is made up of 95% dark energy+ dark matter (of which most is dark energy), and this dark energy is considered to be the main reason for the expansion of our universe. But, anything that is not dark energy attracts each other/ pulls each other due to gravity. But, dark energy is causing the universe's expansion rate to keep accelerating. Can we say this is because it is the opposite of gravity? Instead of the space curving in for objects with mass, does it bulge outwards, preventing things from coming close? Is it right?

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    $\begingroup$ It is dark energy that we believe is driving the accelerating expansion rate of the universe, not dark matter - see en.wikipedia.org/wiki/Dark_energy . And it does not prevent "things from coming close" because its effects only become significant at very large distance scales. $\endgroup$
    – gandalf61
    Commented May 10 at 14:53
  • $\begingroup$ @gandalf61 oo. I guess that is what I meant 😅. But, the fact it becomes significant at large scales makes a lot of sense for what I'm thinking. It probably does not work on short scales because maybe the matter in space cancels the effect of the anti-gravity and its gravity is dominant 🤔 $\endgroup$ Commented May 10 at 14:56
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    $\begingroup$ "dark energy is considered to be the main reason for the expansion of our universe" Not true. Dark energy is responsible for the acceleration of the expansion, not the expansion itself. The expansion would continue just fine without dark energy. $\endgroup$
    – D. Halsey
    Commented May 10 at 21:04
  • $\begingroup$ @D. Halsey--I doubt that it's worth a separate question, but, for whatever it is worth, I believe that the physicist Eric Lerner has (in numerous postings on I.A.I. & various other sites) claimed that the evidence for expansion of the (local?) universe is deficient. He seems to favor a plasma cosmology, based more on electromagnetism (a force) than on gravity (a curvature). I'm curious as to whether some of the aspects of these remarks are consistent with your own views. (Offhand, it doesn't sound like it.) $\endgroup$
    – Edouard
    Commented May 11 at 14:18

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There is a sense in which you could say that the acceleration of the Universe's expansion is a kind of anti-gravity, in that the Universe is behaving effectively like there is a repulsive force between galaxies instead of an attractive force.

However, this is just a words-level description of what is going on in cosmology, and does not capture very well the meaning of the equations that are governing the expansion of the Universe. In particular, there is no new "anti-gravity field" that appears on cosmological scales. Einstein's gravitational field equations describe all of the gravitational phenomena we can observe, from cosmology, to gravitational waves, to gravitational collapse and the orbits of bodies in the solar system. All of these effects are described by the exact same underlying laws, which can be thought of as a changing spacetime geometry and matter moving through that geometry, in a precise way defined by general relativity.

Within general relativity, the reason dark energy leads to an "effective repulsion" on large, cosmological scales, has more to do with the response of the spacetime geometry to an unusual form of matter, than it does with "anti-gravity." In particular, most matter or radiation from standard particles has a pressure which is either small compared to its energy density (in units with $c=1$), or has a positive pressure. Dark energy, on the other hand, has effectively a negative pressure. The response of the gravitational field, to negative pressure, in the situation of cosmology (where the spacetime is approximately homogenous and isotropic) is to have accelerated expansion. It is therefore the response of ordinary gravity to this strange type of matter that drives the acceleration, and not different rules that apply on large scales that change how gravity behaves on a fundamental level. (As an aside, there is a subfield that studies what happens if general relativity does not apply, and a modification of gravity causes the acceleration of the Universe... however, I still would not call this anti-gravity, partly because it does not imply the existence of a repulsive force between point-like objects like planets on smaller, non-cosmological scales).

Another way to look at it, if dark energy is described by a cosmological constant, is that the vacuum is not what we think of as a static, empty space, but a "de Sitter" spacetime which has accelerated expansion. This also isn't really anti-gravity, but a counter-intuitive feature of our vacuum.

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