We don't know…
We don't know how much Universe there is outside our observable Universe.
The observable Universe seems to have a "flat" geometry (in the 3D sense, not in 2D). If it really is globally flat, then it just goes on and on forever, i.e. there is infinite mass. However, just as Earth looks flat on small scales, it might very well be that the Universe has another geometry on scales larger than we can observe.
A recent paper (Di Valentino et al. 2019) claimed evidence for a "positively curved" geometry, implying that the Universe is only (very roughly and with high uncertainties) three times bigger than what we see (see this answer and comments for the calculation). However, the authors sort of cherry-picked one out several of the data sets of Planck Collaboration (2018), and the evidence is not convincing.
…and we don't care
However, no matter if the Universe is finite or infinite, we do not take into account the mass outside the observable Universe in any calculations. This mass does not affect us in any way. Gravity travels at the speed of light, and hence hasn't had time to reach us yet. More importantly, due to the inverse square-law, gravity on any object is by far dominated by nearby sources.
Well, we care a little
There is a sense in which you can say that the gravity is taken into account, though: So-called "cosmological simulations" aim to simulate the formation and evolution of the large-scale structure and the galaxies of the entire Universe. Of course, we cannot simulate the entire Universe, so instead we simulate a large chunk of it; large enough that another similarly-sized chunk would not be statistically different from it (preferably several hundred million light-years on each side).
A galaxy close to the edge of the computational box, say its left side, doesn't only feel the gravity of the galaxies on the right. This would make the simulated universe "implode". Rather, we use a computational trick called "periodic boundary conditions" which means that a particle that happens to travel through the left side of the box enters immediately through the right side. And a galaxy close to the left side feels the gravity of matter lying close to the right side.
Snapshot from the Virgo simulations (Jenkins et al. 1998), where I've tiled the box in several copies, emphasizing the periodic boundaries.