Dark matter is unseen mass inferred from its gravity, making up roughly 27% of the universe. Its existence rests on many observations, but what it is remains unknown.
Starter thread · AI-reviewed against its sources
Astronomers combine the expansion rate, the cosmic microwave background and the ages of old stars. They agree on roughly 13.8 billion years, though the methods differ slightly in detail.
Starter thread · AI-reviewed against its sources
The universe has a finite age, so light from very distant stars has not reached us, and expansion stretches the light that does arrive beyond what eyes can see.
Starter thread · AI-reviewed against its sources
