Shouldnt glaxies be losing mass?

 
Entropy, in physics, doesn't mean growing disorder or disorder at all.. it simply means the number of different ways a system can be arranged. So when talking about the universe and entropy, you first need to determine if you're talking about a finite or infinite universe and if you're talking about a finite universe, if it is an isolated system or closed system. The entropy of an isolated system would never change. So it really depends on what type of universe you're thinking about and if it allows a change in entropy.
 

 
The vacuum of space still contains space.. and we know that empty space isn't exactly empty. There wouldn't be a reason for it to evaporate into nothing as there is still space all around it.. much like how there is space all around us and everything else. Now if you could make a true vacuum that is void of empty space and subatomic particles and everything else, maybe.. but there's a good chance that no such vacuum exists.
 

 
Within galaxies and outside of them, subatomic particles still do come together to form atomic particles over time.. and as they do, they begin to form matter with mass and get pulled in by the galaxy's gravity. So there is always new matter being formed and added to a galaxy and with the way it looks, eventually gets sucked into a super massive black hole and we don't know what happens to it after that. If the black hole consumes it and just adds to it's mass, a galaxy wouldn't lose mass.. only really gain.

 

Gravity is the cummulative pressure of the quantum vacuum or zero point energy. If we view it in this way solves many problems such as the propogation speed of gravity. If the sun suddenly vanished, earth would react nearly instantly, not 8mins later as the sluggish speed of light. There is no such thing as an attractive force, there is only repulsive forces. If we say gravity is an attractive force it raises a few problems. One, either 'gravitins' or 'corpusles' would have to be viewed as intelligent to some degree. Say our vanished sun reappeared, gravitons from earth would instantly know the sun has reappeared and would have to travel from earth to sun and sun to earth. Unless one is to suggest a mass is constantly emitting gravitons, in which case would the gravitons return to earth if they didnt 'find' another mass to attract?

I would say entropy is simply the reabsorption into the ZPE. When a charged mass no longer has the energy to remain a mass its energy is reabsorbed into the ZPE i.e. conservation of energy. The energy does not cease to exist, it is broken down into the most fundamental state. Think of a particle accelerator, when the particles are collided, its all quickly reabsorbed into the fabric of space, ZPE.


Also, you are absolutely right that a vacuum still contains the quantum vacuum flucuations, which means it always contains the fabric of space. If you created a vacuum truly void of the ZPE, you could have no rock in there because the rock is constructed of the fundamental ZPE. Since we cannot create a region void of ZPE, we cannot create a region void of gravity either.

 

Here is an example. The size of the mass and distance between them tells you which waves in the ZPE would be canceled. This creates a low pressure zone between the masses. It appears they are attracting but they are being pushed together by the normal pressure on the outside of the masses and the low pressure between them. So they are pushed together. Why dont they collide? Thats where electromagnetism comes into play.

 

Galaxies gain mass through the absorption of smaller satellite galaxies and other mergers of the sort, in a few billion years we'll be merging ourselves with the Andromeda galaxy. Entropy isn't a concern on scales of millions or billions of years, when they talk heat death of the universe, degenerate universe, and that type of thing they are talking on terms of many trillions of years.
 
There's a video I saw a few months ago which might help to explain entropy a bit. Won't pretend to understand it perfectly myself but it's not what most think it is and not a concern to galaxies, planets and so on on timescale we normally think of.
 
https://www.youtube.com/watch?v=vSgPRj207uE