Could it be that darkmatter is actually the gravitational influence of regular matter that is situated out of the observable universe?
Scientists do not know what dark matter is. It is a placeholder name. It is called "dark" for two reason:
- If it exists, it interacts at most very weakly with electromagnetic radiation. It is "dark" in this sense.
- If it exists, we do not yet know what it is. Science is "in the dark" regarding dark matter in this sense.
Dark matter, whatever it is, and if it exists, is not "regular matter that is situated out of the observable universe". It acts at the galactic scale. A more descriptive name is perhaps "a mystery substance that makes galaxies rotate at rates contrary to the rate suggested by luminous matter". "Dark matter", for short.
I am not denigrating the concept of dark matter. Despite not being able to find candidates for what constitutes dark matter, the alternative attempted explanations for these galactic scale disparities between galactic rotation rates as suggested by luminous (i.e., non-dark) matter are that much worse than dark matter. A more descriptive name for these conjectures is "a mysterious non-substance and non-uniform variation to gravity that makes galaxies rotate at rates contrary to the rate suggested by luminous matter".
Could it be that dark matter is actually the gravitational influence of regular matter that is situated out of the observable universe?
You are perhaps thinking of dark energy, the need for which was only discovered recently. Science is even more "in the dark" regarding the nature of dark energy than it is regarding dark matter. Dark matter was first conjectured to exist in the 1930s to explain the galaxy rotation curve problem. Dark energy was first conjectured to exist in the 1998 to explain the accelerating expanding universe problem.
Just as there are contrarians who dispute the existence of dark matter, there are also contrarians who dispute the existence of dark energy. In both cases (dark matter and dark energy), the contrarians are in the minority. These contrarians do not however fall into the realm of crackpotism, at least not yet. The galaxy rotation curve problem and the accelerating expanding universe problem remain unsolved problems in science at the present time, with the leading candidates having the placeholder names "dark matter" and "dark energy".
If dark matter exists (rather than some strange rewrite of physical laws), then it is required to be all around us. It could not be simulated by the effects of normal matter at great distances.
There are two other things to bear in mind.
(1) If you postulate there is stuff beyond the observable universe then, by definition, it can have no effect on us or our locality. Otherwise it would be observable, since the effects of gravity, like light, travel at the speed of light.
(2) There is other evidence found by combining what we know about the geometry of the universe (e.g., from the cosmic microwave background) and observations from which the primordial big-bang-produced chemical abundances of helium and deuterium are inferred, that most of the gravitating matter in the universe is not "normal matter" made of neutrons and protons.
If, as I suspect, you are actually trying to replace dark energy with matter beyond the observable universe then firstly, the caveat about things beyond the observable universe not affecting us locally still applies.
Secondly, in cosmology you need to use General Relativity to describe the dynamics of the universe as a whole. But, even if we were to adopt Newtonian gravity and consider some "shell" of material around the observable universe; that would not cause a stretching effect, because Newton's shell theorem tells us that the gravitational field inside a uniform shell is zero.
Luminous matter outside the observable universe has been considered as a candidate for Dark Flow, i.e. the unexpected motion in distant galaxy clusters independent of the overall increasing expansion rate of the universe. In my personal model, the increasing expansion is due to the stage of explosion the universe is still in. Using an explosion as a model for motion of the universe as we've observed it, you have various states for any given explosion to consider in order to figure out which one we're in. Roughly, you have the initial rest state, before the explosion has started, you have the peak, before which the rate of expansion of the explosive bubble is still increasing, and after which the rate of expansion of the explosive bubble is no longer still increasing. Simply put, our universe hasn't peaked, and we can't be certain it will. It might just asymptotically approach peak expansion acceleration, so it will always seem like the expansion rate is increasing. As for dark matter being something that makes galaxies rotate faster than expected, my model holds that this too could be due to legacy momentum derived from gravitational interactions with luminous matter outside the observable universe. Think of it as an infinite billiard ball table, and our observable universe as just the circle around us that we can see by standing on it. All the collisions matter to being able to predict the motion of the balls we can see, including those outside our observable circle of balls. Dark matter is only required locally if your model fails to account for the extrapolation of our observable universe as observable from all points in our observable universe, which suggests an infinite universe that is like our observable universe everywhere. Simply consider the causes of the rotational speed of an object, and rather than invent a new cause, use Occam's Razor and infer a cause that occurred significantly far in the past such that while its light could never reach us, its effects are still observable as inputs to the motions of things reflecting light that can.