There are two forces at play in the universe that impact and shape mass. They are gravity, the attracting force, and it's counter, repelling force, absence.
Remove absence from the equation for a moment. Picture the universe as different sized marbles placed on a trampoline. If the quantities of mass in the universe in any one area break beyond a certain level of density, or in the sake of our trampoline, are heavy enough to impact the surface tension of the trampoline... ie are heavy enough to indent it, the mass will pool together in a central point. Eventually, if not for a few potential equilibriums being met, whereby a balance is achieved in the mass being equally dispersed so that all symmetrically attract all others, the entirety of the mass will eventually pool together.
In this model on a universal scale, if gravity is the only attracting or repelling force, eventually everything would pool back together into a Big Crunch.
Remove gravity from the equation instead, leaving only absence, and a universe would simply, eventually coalesce in a state of equilibrium whereby a finite space in the universe is filled equally with mass or an infinite universe spreads mass so thin so as to make it's existence approach zero. In either of these situations, finite universe, or infinite universe mass would no coalesce so much as it would homogenize.
The observable universe, however, is neither of these examples. It is not a place where matter all coalesces into a big crunch nor is it a universe where matter homogenizes into a certain symmetrical density or a density tending to zero. By simple observation, we can see that none of these are true currently.
There are therefore two possibilities.
(1) The balance between gravity and absence is in equilibrium. This means that a constant ebb and flow of the two forces determines what occurs to mass in the universe. You'll find a universe with these rules will have many things in common with our actual observable universe. Matter does coalesce, but it also breaks up, often (cosmologically-speaking) in large explosions. Matter never leaves the cycle. By definition, the universe encompasses all. In a system that is in equilibrium, the conversion of matter to energy... primarily through the life cycle of stars... energy must then also convert back into matter at the same effective rate.
Effective rate suggests that the process from energy to matter may be over a much longer term, but encompass much more mass whereas the process of transformation from matter to energy can be relatively quick or even instantaneous (eg. stars forming versus supernovae exploding). The net result of the equation must be zero, however, if equilibrium is kept. The system can be a function of both time and mass but together must cancel each other out.
In this system, mass bounces over time between matter and energy in a closed system. It is perpetual and completely self-contained. There does exist a logical concern to this, seemingly possible scenario. Equilibrium is hard. VERY hard. The idea that a billion billion billion stars in the universe could all be in equilibrium, while at the same time not being symmetrical is exceedingly difficult, if not functionally impossible. It is certainly statistically improbable. In an infinite universe, the possibility of this occurring would tend toward zero as the number of variables (quantities of mass) tends to infinity. There is another possibility.
(2) There is near-balance between gravity and absence that tends towards equilibrium but never gets there. In this system, much the same as the previous system is true, but the system isn't perfect. The balance is slightly off, and eventually, one side wins. Look back to our examples of removing gravity and removing absence to see what would occur. If gravity has the upper hand, eventually matter wins over energy and the universe ends in a big crunch. If absence has the upper hand, eventually the universe if infinite, will suffer a heat death if finite will suffer homogeneous symmetry--everything would be the same everywhere. Our model can also suffer heat death, if the universe is itself large enough to spread out its mass far enough, though technically it doesn't have to be infinite, although technically that would itself be defining infinite as a quantity by the definition of what the universe is-- i.e "everything".
From what we can see currently, there is at least a lot of interplay between gravity and absence or we wouldn't be here. The longer the universe continues the more we can define the universe as approaching equilibrium. In which way, in fact, we can chart over time any apparent heat loss in the universe as a whole to determine the interplay of gravity and absence, and theoretically could test how close to equilibrium gravity and absence indeed are in our universe.
Be careful however as there is a potential flaw in this logic. If we can prove and define absolute zero as a point at which all motion stops without exception, then we can indeed possibly find a universe that reaches heat death. However, if absolute zero does not, in fact, stop the forces of gravity from attracting mass together, then perhaps even if the universe is large enough for heat death to occur, it may not be possible for it to occur at all. Additionally, it may be possible for it to occur for a period of time or in specific locations, but not actually end the system at all, if absolute zero does not end all motion in practice as it does in theory. This in itself is highly likely. Absolute zero may tend toward being motionless, but the motion may occasionally occur, which means that in that scenario, regardless of the interplay of gravity and absence, the universe wouldn't ever truly end. However unlikely, it would merely tend toward ending--which much data today actually may support.
Observation is key. We must observe with greater clarity exactly what occurs before we can make determinations.
