Cosmologists have given serious thought as to whether our universe is curved rather than flat. In a flat universe, two parallel lines running next to each other will never meet. That seems sensible, but were we to draw two parallel lines on Earth they would eventually coincide at the poles. That is because the Earth is curved.
If the universe were looped, like a doughnut, then in theory if you travelled far enough you would loop around and end up where you began. Light would also do the same thing, so in theory, if the universe were small enough, a big enough telescope would be able to see itself. The light wouldn’t just stop there either, but go around and around like a hall of mirrors.
Astronomers have tested for this effect, searching for patterns of galaxies in the sky that appear to be the same, but have found no evidence for this. That could mean that the universe is not curved at all. Or, it could mean that it is curved, but on scales so large that we cannot see: the expansion of the universe means that there are galaxies we’ll never see beyond our cosmic horizon, as light will not have enough time to travel that distance. If the universe is curved at scales larger than this cosmic horizon, light would never have time to loop back on itself.