- As the universe expands, isn't energy required for stuff to fill in?
If you had a universe with a cosmological constant of the right size and magnitude and nothing else, it is possible to expand, in particular there might be no mass, no matter, no antimatter, no dark matter, no light, nothing except space, time, curvature, and a cosmological constant. You can put the constant on the left and treat it as a geometrical thing (it is constant after all)/or in the right side where energy (e.g. mass) momentum and stress goes which is where for instance matter goes (either dark or regular). If you put it on the left then it is like an exotic fluid that has a balance of energy and pressure that allow it to be more as space expands and be leas if it contracts. The book Gravity from the Ground Up explains this well without too much complicated mathematics.
- How about the past, why was matter all so bunched together in the past?
Given the present you can predict the future (ignore quantum mechanics) but you could also predict the past, if something is moving to the right then you can predict it will be a little bit to the right in a short moment in the future and that it was a little bit to the left a short moment in the past.
All we know is that the universe is as it is so as to make it look to us here and now like it looks to us here and now.
If we see it expanding now then we infer it was smaller in the past. We can try to make theories and models that make this seem more natural but if we do that we only consider theories and models that make that happen.
Basically when you look farther out you see light that left longer ago (took longer to get to you now so left earlier) and it was more dense then, its a fact. Making a theory that fits those facts isn't really much of an explanation, you can turn around as ask why those things work that way.
In Physics we are trying to fit the observations, we observe it was hotter and denser in the past so our theories will be made to fit those facts.
- How did the really dense matter stay that way long ago?
Our theories require that we have places and times for events to happen. We can put down the places now and you could imagine them as a circle or a sphere or mathematical 3d surface in 4d (called a hypersphere). Then for an earlier time we can use a smaller circle or sphere or hypersphere. We can use the ways things are now (the setup on the current sphere) to predict what happens on those bigger spheres (i.e. later) or predict what happened in the past (smaller sphere). In this model time is like the distance between a sphere and the center of all the spheres. So we know where the universe expands, it expands into the future. And it you think about where it came from, it came from the past. And where ever you are your past heads back to that common center. But there isn't anyplace to go when you get there, and why do we even need to insist that the past goes back forever?
All the universe has to do is be a way that allows it to appear to us the way it actually does appear to us. You can try to ask for more and consider many many theories that tell you more but if they all predict the same future for us now how is any one of them better than any other?
- Do people think about before "Before the Big Bang"?
Again, all the universe has to do is be a way that allows it to appear to us the way it actually does appear to us. People do try to ask for more, but not for the sake of answering this question. They consider many many theories that make slightly different predictions for what we'll see in areas we haven't looked yet. So a theory that deals with the earlier universe in an interesting way but allows the current universe to look pretty much like it does now then you check to see if makes some small thing different or predicts something different about something we haven't seen yet. And then you can check them out and try to learn something.
So they ask if it has the possibility to help us learn about the universe.
- If the universe is expanding at an increasing rate, then how are we able to measure any kind of isotropy?
Isotropy is only expected in our current era if you average of truly huge regions. But in the past things seems too isotropic, too uniform. So people theorized that the universe expanded even more rapidly for a while and that expansion helped make it so uniform. So I'm not sure why you are suggesting the opposite.
- Will there not always be local anisotropy everywhere as we are continuously in the process of expansion?
There is local anisotropy because we have moons and asteroids and planets and stars near us nut we've studied how those develop from earlier less extreme versions such as clouds of gas with slightly higher densities so we can trace it back (active area of reach) to exactly how many band what kinds of anisotropies needed to be around in the super distant past. This is like doing our predictions for the future but going backwards in time extrapolating from the present.
- Or is it that the data that we get is from billions of years ago that the isotropy is evened out before we detect it as temperature fluctuations?
I can't tell how one evens out an isotropy, that doesn't make sense. Sorry.