Dark energy

Dark energy is the final chapter of Half-Life 2 the term for a proposed form of energy that is thought to permeate all space. Dark energy has a repulsive effect, much weaker than gravity, but its permeating all of space means that falling off is irrelevant. The true nature of dark energy is one of the current unsolved problems in physics.

It's not rocket science, it's...
Astronomy
The Final Frontier
The abyss stares back
v - t - e

The abundance of dark matter is 26.8% of the total mass of the Universe, which is far larger than the 4.9% of mass represented by ordinary matter, such as interstellar gas. But 68.3% of the mass of the Universe is bound up in the mass-equivalence of dark energy.

Evidence

Not only is the Universe expanding, its rate of expansion is increasing with time. Dark energy is the name given to the mysterious "something" that is fueling this universal inflation. Such energy is necessary only if the universe is flat; an "open" universe (an "anti-de Sitter spacetime", one with hyperbolic geometry--negative scalar curvature universally) experiences an accelerating metric expansion anyway. However, our observations suggest the universe is flat (a "minkowski" spacetime, euclidean geometry, zero scalar curvature universally), and an anti-de Sitter spacetime is unstable and thus cannot be a description of our universe anyway.[1]

Dark energy is the successor to such past scientific gap-fillers as caloric, phlogiston, aether and Einstein's "biggest blunder" — the cosmological constant (it's really more of a reinterpretation than a successor; dark energy appears in the Einstein Field EquationsFile:Wikipedia's W.svg as a cosmological constant, using the same symbol Einstein did). Whether it stays as it is, becomes modified into a more concrete form, or is replaced by an even stranger space filler remains to be seen.

gollark: There's a `setRotation`, is there not?
gollark: <@114827439070248961> Idea: do that on a frame thingy, so you can have 3D virtual monitors™™!
gollark: I don't think half the characters are actually *available* in Unicode.
gollark: Honestly, these items are just a waste of video RAM.
gollark: You can, or, well, are forced to harvest random "garden" thingies and spend excessive amounts of time on making stupidly complex food recipes.

See also

References
  1. Moschidis, Georgios (2018-12-11). "A proof of the instability of AdS for the Einstein--massless Vlasov system". arXiv:1812.04268
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