We only account for 5% of the observable Universe, the mysterious dark energy and dark matter make up the other 95%.
Approximately 69% of the total mass-energy in the observable Universe is dark energy, an unknown form of energy throughout space that is accelerating the expansion of the Universe. We don’t know what it is or where is coming from.
The expansion of the Universe was empirically discovered in 1929, but it wasn’t until 1998 that the first direct evidence for dark energy was found. Prior to that, the only forms of matter–energy known to exist were ordinary matter, antimatter, dark matter, and radiation.
Cosmologists have observed that for the first half of its life, the expansion of the Universe was slowing down, but 5 billion years ago, when the universe was 8.8 billion years old, the mysterious dark energy kicked in to reverse that trend, but we don’t know why.
The mysterious dark energy appears to be an unexplained intrinsic fundamental property of empty space. It has a constant energy density regardless of the volume under consideration, which means that unlike ordinary matter, the energy density of dark energy does not get diluted with the increase in spatial distance.
Dark energy would need to have a strong negative pressure to explain the observed acceleration of the expansion of the universe, it is like a kind of anti-gravity that is blowing the universe apart, but we don’t know much about it.
So far, all attempts to directly observe dark energy in experiments have failed, however, there is indirect evidence of its existence.
The density of dark energy is much lower than the density of ordinary matter or dark matter found within galaxies, however, it constitutes the majority of the mass–energy of the universe because it is uniform and homogeneous across space.
The observed accelerating the expansion of the Universe means that the gravitational repulsion generated by dark energy’s tension driving the expansion is greater than the attraction generated by its energy density, but we don’t know why.
Quantum mechanical equations predict an immense amount of energy locked up in empty space, but the amount astrophysics have actually measured is 10¹²⁰ times less than theoretically predicted. This massive discrepancy is so far unexplained, and could be considered the biggest gap between observation and theory in the history of science.
We still don’t know if the gravitational interaction between matter and antimatter is attractive or repulsive. If matter and antimatter repelled each other, then they would tend to move away from each other instead of annihilating. This could explain why the Universe is expanding at an accelerating rate, eliminating the requirement for dark energy, but we don’t know if this is the case.
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