Why is the universe expanding?
Space itself stretches, galaxies drift apart on a fabric that has grown since the Big Bang.
Space is the thing that grows
The common picture, galaxies flying outward from an explosion, is misleading. Galaxies are not racing through space. The space between them is stretching, carrying them apart like dots inked on an inflating balloon.
Nothing moved across the grid. The grid grew. That is why distant galaxies recede no matter which one you stand on, every observer sees the same retreat.
How we know it is happening
In the 1920s Edwin Hubble measured light from distant galaxies and found it shifted toward red. The farther the galaxy, the stronger the shift, exactly what you expect if space between us is stretching the light on its way over.
Run the expansion backward and everything converges to a single hot, dense beginning roughly 13.8 billion years ago. That convergence is the Big Bang, and the leftover glow of it (the cosmic microwave background) still bathes the sky.
Why it speeds up instead of coasting
Gravity should slow expansion down, pulling matter back together. But in 1998 two teams measuring distant supernovae found the opposite, the expansion is accelerating. Something is pushing space apart faster over time.
Physicists call the culprit dark energy, a property of space itself that resists dilution as space grows. We have measured its effect precisely. What it actually is remains one of the largest open questions in science.