Comment Re:Dark energy discovered 27 years ago?? (Score 1) 90
As far as you know.
Edwin Hubble has discovered that galaxies that are further away are redshifted more, which is odd because one would expect gravitational blueshift.
This correlation between redshift and distance is called the Hubble constant.
One interpretation is that these galaxies are moving away (and that the redshift is a Doppler shift), but that doesn't make sense because at a sufficiently large distance, they appear to be moving away faster than light, and also everything is moving away from everything else. So a better interpretation is that the space between distant galaxies itself is expanding, which has been shown to be the case by the size of the supervoids.
What is not known is why the space is expanding. (A more intuitive interpretation is that the space is constant and everything in it is becoming smaller. But if we use ourselves as reference, we are not becoming smaller relative to ourselves. Space is growing relative to us. The observable universe is expanding.)
This expansion corresponds to a factor (the cosmological constant) describing the accelerating expansion of the universe. And across cosmological time and distances it is not constant, there is some form of energy at work.
It is called "dark energy" (ever since 1998, 27 years ago) because nobody knows what it is. But it is not conjecture, there is definitely something going on.
And that has been discovered, shown, tested, and proven, which makes it theory. Purely descriptive, of course, but proven theory nonetheless.