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.