In 1929, astronomers discovered something very surprising.
The universe was expanding.
In the years that followed they reasoned that if it was expanding, then if you went back far enough in time, then you should find a single point of origin followed by an explosion.
This is the idea behind the Big Bang.
But they were in for another surprise. They later found that this expansion was accelerating.
That’s pretty crazy. No normal explosions accelerate.
Some physicists found that if you adjust our models of what the universe is by adding one of two things, they match the acceleration that we’ve observed. Either:
- The entire universe filled with a type of ‘smooth’ background energy (called the ‘cosmological constant’),
- The entire universe is filled with a type of ‘bumpy’ background energy (called ‘scalar fields’).
Both of these are called ‘dark energy’.
It’s called ‘dark’ because we can’t see it. It only seems to affect the force of gravity. It can be thought of as background energy that ‘lubricates’ the expansion of the universe (it’s not a perfect analogy, but it’s close).
From the distortions in gravity that we can see, we can calculate the amount of energy that dark energy has.
It has to have a very low density, so low that it is imperceptible to all of our current equipment. But because it stretches out across the entire universe, the total amount of energy in dark energy is phenomenal.
If our predictions are correct, then the energy content of the universe is 68.3% dark energy, 26.8% dark matter, and just 4.9% ordinary matter (ordinary matter is the galaxies, stars, planets, and everything on them). For this reason, we have to understand dark energy if we want to understand the universe.
This adjustment aligns our theories with our observations, but we have not yet discovered any evidence that this energy actually exists, or whether it takes the form of a cosmological constant or scalar fields. Its exact nature is one of the biggest unsolved questions in physics.