Is our universe expanding? Or does it only seem bigger because our ability to look further into the universe has grown?
It is expanding.
Dark matter is causing the universe to expand and it has been observed to happen as we've seen galaxies move farther apart. Gravity is what keeps galaxies together
The Universe is definitely expanding.
It's not a matter more and more powerful telescopes seeing more and more distant galaxies. Instead we measure the expansion of the Universe from the spectra of galaxies showing a red shift -- the galaxies are moving away from us and that causes the light from the galaxies to be stretched out to longer wavelengths and lower frequencies. The red shift is what shows the expansion, not the absolute distance.
1) The doppler effect for how moving objects lead to waves having a different frequency: https://en.wikipedia.org/wiki/Doppler_effect
2) How this effect is seen as a red shift when galaxies move away from us: http://astronomy.swin.edu.au/cosmos/c/cosmological+redshift.