A White Dwarf star is a remnant of a star like our Sun that has exhausted it's nuclear fuel. It is primarily made of Carbon, the ash of the Helium fusion that powered the death throes of that dying star. In it's final act the star throws off its outer shroud exposing the Carbon core.
Some White Dwarf stars orbit a massive red giant from which it siphons material. As the mass of the White Dwarf increases, due to the infalling material, the temperatures and pressures within it grow. When it passes 1.44 times the mass of the Sun, all the Carbon will fuses at once, releasing vast amounts of energy that completely destroys the White Dwarf star, spreading it's guts all over the sky.
This is a type 1a supernova, and it will shine five billion times brighter than the Sun for several weeks. It is also precisely tuned. That 1.44 solar mass threshold means that every type 1a supernova is as bright as every other.
When the brightness of an event is predictable, we call it a standard candle. Since we know exactly how bright it is, we can calculate it's distance by measuring how bright it appears to us here on Earth.
In the late 90s a team of researchers studied a large number of these events, reaching extreme distances, and therefore extremely far back in time. By measuring the red-shift of the spectra of those events they calculated how fast they were being carried away from us by the expanding universe -- and they discovered something very strange.
We expected that the mutual gravity of all the galaxies, and all the matter in the universe should be slowing the expansion of the universe down. After all it takes energy to climb against a gravitational field; and that energy loss must be slowing things down.
What the researchers found, instead, is that the expansion of the universe is speeding up. This means some kind of energy is being pumped into the universe that opposes the inward tug of gravity and pushes the galaxies away from each other ever faster and faster as time goes by.
We call that new energy "Dark Energy" -- and we have no idea what it is or where it comes from.
Source: x.com/unclebobmartin/status/1830631675304230952