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Why can't I find anyone mention this online, what causes people to dismiss this theory instantly, so it's not even worth asking?

So "dark gravity" is gravity that is pulling all planets away from each other, even though the force each planets excerpts on each other should pull them inwards, towards each other.

Can dark gravity be as simple as a lot of planets outside the observable universe having a total gravitational force that is much greater than the forces pulling everything together, causing everything to spread apart?

I know gravitational pull decreases with distance squared, so the force on a planet outside the observable universe on Earth for example would be very minimal. But as we don't know the size of the universe, couldn't there be an infinite amount of planets, having a total gravitational sum that results in a greater force than the one pulling everything together inward?

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    $\begingroup$ I don't understand why you can't find any mention of this online. When I Google "dark gravity," it brings up 9,570,000 results. $\endgroup$ – D. Halsey Mar 11 '18 at 22:26
  • $\begingroup$ I assume this is intended as an explanation for dark energy/accelerating expansion of the universe? The problem is that adding more mass to a universe slows its expansion, regardless of whether it has a finite "radius" in a closed model or is spatially infinite. So one reason for the dismissal might just be that the theory doesn't work? $\endgroup$ – Anders Sandberg Mar 11 '18 at 23:59
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    $\begingroup$ Since matter in the unobservable part of the universe is racing away from us faster than the speed of light we can't see it. Likewise gravity which also travels at the speed of light cannot be felt from the unobservable region. $\endgroup$ – Lewis Miller Mar 12 '18 at 15:14
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I'm sure you mean something big like "galaxy superclusters" rather than something really tiny (at cosmic scales) like "planet" in your question. You also seem to have confused your terms: at macro scale, the effect you describe of "pulling all planets [sic] away from each other" is simply the expansion of space.

What we observe is that at smaller cosmic scales, gravity is the dominant force: planets are bound to their host star, stars are bound within galaxies, galaxies are bound within clusters, etc. However, we also observe that space is expanding, and the greater the distance, the greater the rate of expansion. At very large scales, this expansion pulls things apart more than gravity draws them in. In the 1990s it was observed that this rate of expansion isn't steady but is accelerating, and "dark energy" is the label we use to explain this, although what "dark energy" actually consists of is still a matter of intense research and debate.

Regarding "dark gravity", you might be interested in this 7-minute Youtube video in which Neil deGrasse Tyson says "dark matter" is a misnomer and recommends we call it "dark gravity" (or even "Fred"). The specific point he makes is that a majority of the gravitational effects we observe at galactic scales cannot be explained by the mass/matter we calculate to exist, and it's misleading to propose that the explanation is dark matter when we really have little understanding of what it might be.

Erik Verlinde has proposed an alternative theory called "emergent gravity (EG)", in which he posits that "the observed dark matter phenomena are a remnant, a memory effect, of the emergence of spacetime together with the ordinary matter in it." Or, put more simply in a 2016 paper by Margot Brouwer et al, "In this theory, the standard gravitational laws are modified on galactic and larger scales due to the displacement of dark energy by baryonic matter." Brouwer's research was a first test of EG using weak gravitational lensing; it found that "the prediction from EG, despite requiring no free parameters, is in good agreement with the observed galaxy–galaxy lensing profiles."

Regarding your question about "planets [sic] outside the observable universe", you might be confusing this with theories that involve multiple universes or other dimensions. If the Universe is flat, homogenous and isotropic - and all the evidence appears to offer increasing support for this - then any gravitational effects from beyond the area of the Universe we can observe would be evened out, so it's most likely not the case that what's outside the observable universe is behaving differently to what we can observe.

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