In other words, Gamow predicted that our Universe began with a hot Big Bang, and that plasma once filled all of space. What’s exceptionally interesting about this is that the prediction is testable: whereas cold hydrogen gas is transparent and invisible, hot hydrogen plasma is opaque and glows brightly, like the surface of the Sun. This means that when we gaze ever farther into space as in Figure 3.3, we should encounter old galaxies nearby, then young galaxies beyond them, then transparent hydrogen gas, then a wall of glowing hydrogen plasma. We can’t see beyond this wall, because it’s opaque
