OT: The Universe's "Dark Energy" Explained?

What is dark energy?  Rough attempt to provide a (non-rigorous) answer:

We are familiar with normal matter. We, our dogs, our football stadiums, and the dark side of the moon are made of normal matter. As Newton realized and Einstein clarified, normal matter is the source of gravity. All matter attracts all other matter. If you know how much a football or a star weighs, and you know how fast it's moving, then you can use physics to predict how the force of gravity influences its motion and predict its path of travel.

At first, we assumed that stars and all the not-yet-made-into-stars "stuff" in galaxies was also made of normal matter. And then scientists began to observe stars in nearby galaxies closely, and watch how those stars moved. (Roughly, how they orbit the centers of their galaxies.)  And it was quickly realized that if you add up all the stars you can see and how much mass they must have, and then you watch the stars move, you find that the mass and the motion don't match. There must be something like 5x more mass there that we can't see in order to explain the motion. Thus, the term "dark matter" was coined to explain stuff that must create gravity, but doesn't shine brightly in the sky for us to see.

Turns out this "dark matter" theory actually cleanly solves many different problems in cosmology (not just the galaxy rotation example), so even though we don't really yet know what "dark matter" is made from, the idea that it exists is at this point uncontroversial.  (Note that while we don't know what dark matter is, we do what it's not. It's not a bunch of planets or comets or black holes. If there were 5x more black holes than stars in the universe, we would definitely be able to tell!  Seems clear that dark matter is not made of normal matter.)

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Okay so separate from all of that, it's quite clear that the universe is expanding. This has been known since the late 1920s, and Einstein famously could have predicted it when he first developed General Relativity but chickened out. (The guy who did first find it, Hubble, got a rather nice telescope named after him for his efforts.)

The expansion of the universe began with the infamous Big Bang. (A point of common confusion -- the idea that the universe is expanding tends to suggest that it used to be smaller, which often leads to people saying that "the whole universe was once the size of a grapefruit" and leads people to ask where did the Big Bang happen?  The answer is that the Big Bang happened everywhere, and the Universe might well be infinitely big now and might well have been infinitely big then too.  The thing about infinity is that you can multiply it by a very large number and it's still infinity. The part that used to be grapefruit-sized is just the part we can see, the observable universe, which we're pretty sure is only a small fraction of what's actually "out there.")

Remember how earlier we said that if we know how much something weighs, and we know how fast it's moving, we can predict where it will go?  We can do this with the expansion of the universe as well. We know how much various galaxies weigh, and we can watch them move, and we can predict how gravity will slow down the expansion of the universe over time as all matter is attracted to other matter.

But there's a catch. It's become increasingly clear from recent observations that the expansion of the universe is NOT slowing down. In fact, it's speeding up! This is bonkers. There is no known mechanism to drive this acceleration. In General Relativity, positive mass has positive (attractive) gravity, and mass and energy are interconvertible (E=mc² and all that), so when you see something that looks like repulsive gravity, the equations of General Relativity give you something back that looks like a negative energy.  Since we already have "dark matter" to explain one mystery, the name "dark energy" was coined to describe this mysterious repulsive force that apparently fills the universe and accelerates its expansion.

So, if you are confused about what "dark energy" is, you're not alone. There really is no current explanation for what it is, other than that it's a term in an equation that allows us to fit the current best observations for how the universe is changing with time.

The current paper attempts to begin to explain what dark energy is, and in the process invokes the unexpected twist that maybe as the universe expands, the black holes inside it are expanding with the universe (not due to gobbling up stars or anything; they're being dragged to larger size by the growth of the space they sit in.)  That is a fun new twist. Normal matter does not expand with the universe. You and I and Jim Harbaugh and the Andromeda galaxy are not expanding with space. The Earth is gravitationally bound to the Sun, and they will stay the same distance apart even as the universe around them expands (nor is the Earth itself expanding). But black holes have always been weird, so them being weird in this way is perhaps not that surprising after all.