Dark energy is usually portrayed as something mysterious, because mystery sells. But if you’ve read what Einstein said and got the gist of gravity, it isn’t mysterious at all. What is, is why Einstein didn't predict the expanding universe. But anyway, on page 185 of The Foundation of the General Theory of Relativity, Einstein said this: "the energy of the gravitational field shall act gravitatively in the same way as any other kind of energy". That’s "dark" energy. And it’s right there in the room you’re in, right in front of your face. Let’s tuck you out of harm’s way, then take the air out of that room. Then we can line it with lead to keep out the cosmic rays, and make it cold and dark so there’s no particles in there. But there’s still a gravitational field in there. There’s still energy there. Some will say there’s virtual particles there, but there aren’t, not in any real sense. Check out Matt Strassler’s website and note this: "A virtual particle is not a particle at all". Virtual particles are like accounting units, like you divide the field into squares and say each is a virtual particle. They aren't real particles. They don’t actually pop in and out of existence. Hydrogen atoms don’t twinkle. Magnets don’t shine. There are no particles in your cold dark empty room. But there is energy there. And it is dark. Please Register or Log in to view the hidden image! Image credit: NASA Some will say it’s just field energy, but they aren’t giving you the whole story. For that, you have to read Einstein talking about field theory in 1929. He was referring to electromagnetic fields and gravitational fields, and he said this: "It can, however, scarcely be imagined that empty space has conditions or states of two essentially different kinds". Did you catch that? According to Einstein, a field is a state of space. It isn’t something separate to space, it’s a state of space. So field energy is really spatial energy. The energy of the gravitational field is the energy of space itself. You can’t see this energy. It isn’t made up of particles. The only thing that’s there is space. And like Einstein said in his Leyden Address, space isn’t nothing. It’s really important to appreciate this. And the way to do that, is to appreciate that waves run through it. If you’ve ever looked at displacement current you’ll know that Maxwell talked about transverse undulations. A ripple in a rubber mat is a transverse undulation. It’s a transverse wave. So are light waves. So are gravitational waves, which you can read about at LIGO. See where they talk about the two arms of the interferometer forming an L shape? See where they say "if the two arms have identical lengths”? The thing about waves is this: if a seismic wave moves through the ground, the ground waves. If an ocean wave moves through the sea, the sea waves. And if a gravitational wave moves through space? Space waves. Yes, space waves. And when it does, the two arms of the LIGO interferometer are no longer the same length. If you’ve got a rubber mat, give it a shake and watch the wave run through it. A wave in space is like that, but in a three-dimensional bulk. Like I was saying, space can be likened to a gin-clear ghostly elastic jelly. To emulate a gravitational field you insert a hypodermic needle and inject more jelly to represent the E=mc² energy of the Earth. The surrounding jelly is then pressed outwards, and now there’s an energy-pressure gradient in it. Because of this “the speed of light varies with position”. And because of that, light curves. That’s what Einstein said. Light curves like a car veers when it encounters mud at the side of the road: Please Register or Log in to view the hidden image! Note that the squares are flattened where the gravitational field is. And notice that I said space is like some gin-clear ghostly elastic jelly, and that you inject more jelly to represent energy. It’s like energy and space are the same thing. Take a look at the clear night sky. Don’t look at the moon or the stars. Just look at the space between those stars. Dark, isn’t it? What are you looking at? Dark energy. And get this: space isn’t just some gin-clear ghostly elastic jelly with pressure gradients in it. The whole shebang is under pressure too. It’s like the whole universe is some squeezed-down stress-ball, only you’ve opened your fist. So it expands, like the raisin-cake analogy, something like this: Please Register or Log in to view the hidden image! Note that the squares aren’t flattened, so there’s no overall gravitational field in the expanding universe. And remember what Einstein said: a gravitational field is a state of space. It isn’t something that sucks space in, so the universe was never going to collapse under its own gravity. Also note that conservation of energy tells you that the cosmological constant can’t stay constant over time, because it’s "the energy density of the vacuum of space". This is obvious once you know about "the strength of space", which is mentioned on page 5 of this paper. If you’ve ever watched the Discovery Channel you’ll have seen the balloon analogy. Think "bag model" and remember the tension. Think of a balloon in a vacuum. A balloon is the size that it is because the pressure inside is balanced by the tension in the skin. But the expanding universe is likened to an inflating balloon, where air is being added. That's not good, because adding air is like adding energy, and conservation of energy is one of the most important laws of physics. A better analogy would be a bubble-gum balloon. Or a balloon made out of silly putty. The skin relaxes a little so it expands a little. But as it expands, the skin gets thinner and weaker. The tensile strength reduces. The dimensionality of energy is pressure x volume. The pressure drops, tension is negative pressure and that drops too, but the volume increases so energy is conserved. Only the balloon doesn’t stop expanding, because the skin gets weaker and weaker. Instead it expands faster and faster. It’s all pretty simple when you see it right. You appreciate that the pressure and shear stress in Einstein’s stress-energy tensor is there for a reason. Because space really is like some ghostly gin-clear elastic. You also appreciate that it's the stress-energy tensor because stress is directional pressure and it isn't a million miles away from Kip Thorne's elastodynamics. You appreciate at last that energy tells space how to curve because it's like injecting jelly into jelly. Yes, it's all pretty simple when you appreciate that you can't separate space from energy. And that there's a lot of it about. And that space, of course, is dark.