This is a good question but I don’t know that much about it I’m afraid, although I now really want to find out more! Most scientists today believe the universe has been expanding since the big bang. Not everything in the universe – just the space between everything. There’s still a question over what will happen to the universe after this expansion. It all seems to depend on a mysterious object called dark matter. We don’t really know what it is but it explains why lots of objects in the universe behave the way they do, contrary to how we think gravity should affect them. As far as I know (and I could be wrong) scientists aren’t sure if this dark matter will accelerate the expansion or cause the universe to crash back in on itself. We can study things like supernovae to try to find out more about how the universe is expanding. Whatever happens, we’ll have a long time to make new discoveries – billions and billions of years!
There are 3 options for what the universe is doing: it is expanding (and has been since the Big Bang), it is shrinking (heading towards a Big Crunch, after an expanding phase maybe?), or it is the same size and always has been and always will be.
The numbers between the three are not very large and by the time you have error in the measurements, it’s really hard to work out which one it is.
Current theory is that we are in an expanding phase. It could keep expanding, or it might get to a certain size and then start shrinking back again.