Transcript:

Let's look at dark energy. Here's a fact. Everything, absolutely everything that is above 0 Kelvin, which is absolute zero — on a centigrade scale, that is −273.15°C, or 0 K — absolutely everything that is above 0 Kelvin in temperature emits light. Light that is emitted is not in the visible spectrum; it's in the infrared.

Everything emits light except dark energy. Dark energy does not emit light, which is why it is called "dark." It does not emit light. But the presence of the dark energy is felt.

Why is it felt? I mean, how do we know it's felt? Because of the expansion of the universe. It is expanding faster today than it was in the past. And it can only happen if there is something that is dark, which is not emitting light, but has the energy to push the expansion of the universe. This is dark energy.

Dark matter, on the other hand, is completely transparent to light. It does not interact with light at all.