The Dark Universe: What We Still Don't Understand

If everything we can see is only 5% of the universe, what is the other 95%? The intellectual-honesty module.

Here's the twist the whole course has been building toward: everything we've celebrated so far — every star, planet, galaxy, and you — adds up to just 5% of the universe. The other 95% is two things we have named but cannot explain. This is the intellectual-honesty module, where modern science says, out loud, that we don't know — and means it.

We'll meet the evidence for dark matter (the invisible scaffolding that keeps galaxies from flying apart), the shock 1998 discovery of dark energy (the mysterious push accelerating the universe's expansion), and the cosmic web that ties the visible and invisible together into the largest structure that exists. Far from being a downer, the unknown 95% is the most thrilling frontier in all of science.

Dark Matter: The Invisible Scaffolding of Galaxies

Galaxies spin far too fast to hold together — unless something invisible, and five times more abundant than everything we can see, is holding them.

45m

Dark Energy and the Accelerating Universe

In 1998 two rival teams set out to measure how fast expansion was slowing down. It wasn't slowing down at all.

45m

Cosmic Architecture: Clusters, Filaments, and the Cosmic Web

Zoom out far enough and galaxies stop being islands and become droplets on a web of filaments — the largest structure that exists.

30m

What you'll carry out of this section

The evidence for dark matter — and that it outweighs ordinary matter about five to one.
Dark energy and the accelerating expansion, plus the honest 5% / 27% / 68% cosmic inventory.
The cosmic web — how gravity and dark matter sculpted the largest structure in existence.
A genuine feel for the edge of human knowledge, and why an open question is exciting rather than embarrassing.