Err... what supernova? "With our telescopes, we can see the supernova advancing, unstoppable, destroying everything it touches" where - some evidence please? "just like they were created, they will be destroyed once again" yes, that makes sense.

Within the Milky Way proper, the vast majority of planets are going to form within the disc, so your night sky will always have the edge-on disc going through it, even if the planet was on the edge of where we define disc. Your best bet might be to live on a planet that formed within one of the Milky Way's satellites…

Stars and gas can be 'stripped' out of galaxies, and this might in part explain the background. For example, a galaxy in a rich cluster of galaxies can have loosely bound stars and gas removed as it passes through the dense 'intracluster medium', both from interacting with the hot gas present (which buffets the galaxy…

Good question. There's a fuzzy region in parameter space where we have to be careful about what constitutes a galaxy (a bit like 'is Pluto a planet'). Small clumps of dark matter could arguably accrete a bit of gas and form a new dwarf galaxy. But the bigger haloes of dark matter that can 'host' a galaxy like our…

The large scale structure of the universe, including voids, is really determined from gravity that shapes the underlying distribution of dark matter. As dark matter clusters together into a hierarchical structure (from single clumps to massive clusters), the voids naturally form (if you put more matter somewhere, you…

It might not be required to actually *form* a galaxy, but the presence of a central black hole appears to be very important for the evolution of galaxies: there is a strong correlation between the mass of the central black hole and the mass of stars in the bulge surrounding it, despite many orders of magnitude…

My gut feeling is that the extragalactic stellar light is the result of stripped or ejected stellar material. We know in clusters of galaxies for example that gravitational and hydrodynamical processes can remove loosely bound stars and gas from galaxies and these end up in intergalactic space. Stars will just diffuse…

Indeed, there are a lot of unknowns in this field (dark matter and dark energy being chief among them)... however, I would stress that the current cosmological model we have to describe the Universe and its contents does a *very* good job of describing empirical data on a wide range of scales, from the CMB to the…

For very distant galaxies we can judge the distance using the redshift effect: galaxies are moving apart from each other, and the ones that are far away are moving away from us faster. Like the doppler effect that causes a police siren to change pitch as it zooms past you, so light is moved to longer wavelengths for…

Whether galaxies merge depends on if they can become gravitationally bound - often, this is determined by the speed of the collision. Two galaxies with very high relative velocities (as you might find in a cluster) may just fly by each other, and not become bound (although there might be some gravitational…