Advanced civilisations are hungry for energy. Our own is estimated to use $$6 \times 10^{20} J$$ a year, and that number grows by roughly 1.1% every year.

How could an advanced civilisation tap into a source of energy that could feed its needs? The mathematician Freeman Dyson proposed the idea of large spherical structures that could be built around stars, to capture the star's radiation.

The sun emits around $$3 \times 10^{28} J$$ every minute, a hundred million times more than we use in a year. Capture a big enough chunk of that with your Dyson Sphere, and you could easily feed a trillion people. So the idea does sound promising.

This video describes Dyson Spheres in more detail.