In Scientific Controversies, hosted by our own Janna Levin, we tackle complex, conceptual, occasionally amorphous topics like animal consciousness and string theory. These rich, hour-long conversations take place in person at Pioneer Works, and feature big thinkers exchanging on big questions. To watch is a feast. “5 Takeaways” is a snack, an amuse-bouche for the mind. Because comprehension sometimes demands—or, at the very least, appreciates—distillation, and the internet loves a listicle. Below, for the benefit of lay science enthusiasts, contributor Anil Ananthaswamy serves up key takeaways from two conversations: one on dark matter, with guests Elena Aprile and Peter Fisher, and another on dark energy, with guests Rachel Rosen and Pedro Ferreira.

The universe has a dark sector. What is it?

It’s easier to start with what it is not. It’s not normal matter, which is everything that’s familiar: we and all life on Earth, the solar system, the Milky Way, the hundreds of billions of stars in our galaxy, the hundreds of billions of other galaxies in the universe, and the gas and dust strewn across intra-galactic and intergalactic space, all of it, down to the last elementary particle. This normal stuff makes up about five percent of the universe. The other 95 percent comprises the dark sector, named so partly because we don’t know its exact nature and partly because it doesn’t interact with light—which technically makes it invisible, not dark. The dark sector has two major components: dark energy and dark matter.

Why do we think there’s dark matter out there?

Astronomers have known since the late 1930s that something was amiss about the speeds at which galaxies move around in clusters, or the speeds at which stars and gases rotate about the center of a galaxy. Given the gravity of the discernible, luminous matter, the cosmic objects should be moving much slower than they are. By 1970, astronomer Vera Rubin had accurately measured such anomalous speeds in the nearby Andromeda galaxy. Her observations suggested that the gravity of an unseen mass was holding the galaxy together, preventing it from flying apart. This unseen mass has been termed dark matter. It doesn’t interact electromagnetically with normal matter, meaning it doesn’t emit nor engage with light. Multiple methods of estimating the amount of dark matter show that it makes up about 27 percent of the universe.

But what is dark matter?

We don’t know. Astrophysicists once thought that dark matter could be non-luminous normal matter that’s hard to see. These would include: black holes; or neutron stars, which are the collapsed cores of supergiant stars; or brown dwarfs, which are small stars that cannot sustain the nuclear fusion needed to emit stellar light. These are collectively called massive astrophysical compact halo objects (MACHOs). Astronomical surveys, however, failed to find anywhere near enough MACHOs for them to be dark matter. Now, physicists, for theoretically plausible reasons, think dark matter is most likely made of particles known as weakly interacting massive particles (WIMPs), which would interact with normal matter only via the weak nuclear force and gravity. Efforts to create such particles at the Large Hadron Collider (LHC) at CERN near Geneva, Switzerland, have failed. It’s possible that their mass is beyond the reach of the LHC, and creating them would require more powerful particle smashers. Also, efforts to detect potential collisions of dark matter particles with nuclei of normal matter using ultra-sensitive detectors housed in deep underground mines have also failed. But the circumstantial evidence that dark matter exists is so strong that cosmologists continue to look for any clues as to what it might be.

What about dark energy?

If dark matter poses problems, dark energy compounds them. Through the 1980s and ’90s, cosmologists studying the expansion of the universe were expecting to find that spacetime had been steadily expanding for about ten billion years after the big bang, but this expansion would eventually have been reined in by the gravity of all the matter in the universe. But instead of slowing down, the universe’s expansion seems to have started accelerating about four to five billion years ago. Mostly, physicists attribute this to the inherent energy of spacetime, and call it dark energy. As the universe expanded, the amount of overall dark energy increased beyond a certain threshold, causing spacetime to blow apart—in a manner of speaking. Plug in the right amount of dark energy and dark matter into Einstein’s equations of general relativity and you can accurately model the universe’s observed initial unhurried expansion and then its recent acceleration. In today’s universe, dark energy makes up about 68 percent of the cosmos.

So why does dark energy pose problems?

For starters, dark energy seems to have just the right value, in terms of the amount of energy per unit volume of space. Had it been less, the universe would have long since collapsed due to the gravity of matter; any greater and the universe would have blown apart before the stars and galaxies and planets could form, let alone intelligent planetary life pondering the mysteries of the cosmos. Physicists struggle to explain why dark energy has the value it does. It frustrates them that they cannot derive it from first principles. Also, quantum physics says that the emptiness of spacetime is not really empty: it’s a froth of virtual particles that constantly appear and disappear; this process gives spacetime an energy that’s about 120 orders of magnitude (=10¹²⁰, or a trillion raised to the tenth power) greater than the observed value of dark energy. Something isn’t adding up. That’s why almost all the current and planned cosmological and astrophysical surveys of the cosmos are focused on deciphering the dark sector. ♦