The Great Filter is the idea that, in the development of life from the earliest stages of abiogenesis to reaching the highest levels of development on the Kardashev scale, there is a barrier to development that makes detectable extraterrestrial life exceedingly rare. The Great Filter is one possible resolution of the Fermi paradox.
https://en.wikipedia.org/wiki/Great_Filter
The Fermi paradox is the discrepancy between the lack of conclusive evidence of advanced extraterrestrial life and the apparently high likelihood of its existence. As a 2015 article put it, “If life is so easy, someone from somewhere must have come calling by now.”
https://en.wikipedia.org/wiki/Fermi_paradox
Personally I think it’s photosynthesis. Life itself developed and spread but photosynthesis started an inevitable chain of ever-greater and more-efficient life. I think a random chain of mutations that turns carbon-based proto-life into something that can harvest light energy is wildly unlikely, even after the wildly unlikely event of life beginning in the first place.
I have no data to back that up, just a guess.
That’s something really interesting, though. When we look at distant objects, we aren’t limited by the distance they’re at right now. We’re limited by the distance they were at when they emitted the light.
So the observable universe is still growing because the edge of that bubble is such a long time ago that everything was still much closer together.
But the light that reaches us is constantly getting stretched (red-shifted), so I’m not sure that our bubble is growing. Instead when they’re stretched too thin, we won’t be able to see it. I’m not 100% sure on the expansion rate of the universe and the pace of red shifting. Also, eventually all the galaxies are expected to be pushed so far away from each other due to the pressures exerted by Dark Energy, that soon we’ll only be able to see just the stars of our Milky Way.