The word “dark” in both dark energy and dark matter means unknown. We do not have any firm idea what either of them is. We have a number of discarded hypotheses which they definitely are not, and a number more hypotheses that they might be, but we haven’t proved. We invented both because otherwise our sums don’t add up. Which means that why are both essentially failures of modern physics – places at which science has to admit it doesn’t know.

Dark matter is a real substance, whose composition remains unknown that has significant mass to keep galaxies together. Its presence can be observed and mapped. Among other things, it keeps the stars in a rapidly moving, spinning galaxy from flying apart. It also keeps clusters of galaxies moving more or less together. Its presence is inferred from its gravitational effects.

Dark energy is a force normally over, come by the effects of gravity. As the universe expands, space grows thinner at the edges of the Universe. Dark energy begins to push things apart because it can now overcome the effects of gravity. So, rather than the expansion of the universe, remaining constant, or slowing down, it’s actually increasing along the edges.

When we find out their nature, when we have a possible answer to this question, we will give them new names according to their nature. Or we will chuck them out as an embarrassing mistake, like phlogiston and the ether.

Dark energy acts as if (or makes gravity act as if) empty space was repelling everything else. Small stuff can resist that repulsion through strong force, electromagnetic, and gravitational attraction. But things bigger that galaxy clusters don’t have enough attraction to do that, so they’re all carried away from each other.

Our universe seems to have had exactly, or at least almost exactly, the energy density to expand forever, but slower and slower over time. But because of dark energy, that expansion is accelerating faster and faster, ever since dark energy became dominant over matter a few billion years ago.

Dark matter just provides for gravity to work on. The way it’s distributed—roughly spherical blobs around each galaxy, strung out on huge filaments—drives most of the large-scale structure of the universe.

Dark energy is the kinetic energy of galaxies and clusters due to the relativistic speed of expansion, plus the potential energy, measured with respect to a reference point, plus the pressure and volume of the matter bearing part of the universe. This pressure is what makes the universe to accelerate its expansion.

The above means that the premises general relativity is founded on are wrong because it is wrong to extrapolate the way the observable universe expands to the entire universe. Thus the universe is not isotropic and homogeneous, it has a center and a boundary; clusters and galaxies have inertia with respect to space; and there is a point of reference the universe is expanding from, the singularity at the big-bang.

Therefore the universe will expand for ever, it will never collapse, and it will do it at less than the speed of light.At its boundary, gravitation is expanding at the speed of light, it cannot be reached, and time is not ticking.