When we try to picture the cosmos, we tend to only think about the shiny stuff — stars, galaxies, and very very hungry black holes. But stars cannot burn their hearts without cold hydrogen to collapse from, cannot coalesce into galaxies without dark matter to pull them together, and cannot shine as black holes after death without more gas and dust to feed on. The background medium, as it turns out, has as much tremendous importance to astronomy as the objects underneath the spotlights.

The stuff between stars

Let’s start with our home galaxy, the Milky Way. There are approximately a hundred billion (10¹¹) stars bound in its gravitational system, collectively weighing around 10¹¹ Suns (our Sun is pretty average). According to our current model of stellar evolution, every single one of them started out as a loose (one might even say nebulous) cloud of gas and dust, contracting for millions of years until their core reaches the critical temperature for nuclear fusion to occur. Then, after anywhere from millions to billions of years, they exhaust all their fuel, and through either a gentle winding-down or a spectacular explosion, return most of their matter back into the medium.

When the births and deaths of stars roughly balance, it leaves around 10¹¹ Suns’ worth of gas and dust hanging around inside the galaxy, on the same order of magnitude as the collective mass of the stars themselves. This is the visible portion of the interstellar medium (ISM), i.e. the stuff between stars; Its cosmic cycle between loosely-bound clouds and dense, burning stars forms the basis of the galactic ecosystem. They are visible, because even though they don’t shine like stars, some still glow from the heat of gravitational contraction or the radiation of nearby stars, often at very specific colors (known as emission lines in the light spectra). Their absorption of starlight behind them can also noticeably dim their background, again, often at very specific colors (known as absorption lines). We are able to map out where these gas and dust are by looking for their distinctly colored emission lines and absorption lines. Unsurprisingly, they’re mainly concentrated where most of the young stars live: in the thin, flat central slice of the Milky Way disk. (Our Sun lives here too, which explains why we can’t really see the center of our galaxy — the pesky dust blocks the line of sight.)

Now that we have stars, we need a way to put them into a galaxy. You might think that, mirroring our Sun’s role in the solar system, having a super massive black hole at the galactic center would be enough to hold everything together. Unfortunately, Sagitarrius A*, the super massive black hole in the core of the Milky Way, just isn’t supermassive enough! While the Sun holds over 99.8% of the mass of the solar system, Sagitarrius A*, weighing at a measly several million Suns, is less than 0.01% of the mass of the Milky Way. Adding the bulge of stars, gas, and dust clustered around Sag A* doesn’t make that big of a dent either. For galaxies to form, there has to be a lot of extra mass concentrated near the center that we’ve so far not accounted for.

This is the stuff we call dark matter — the hidden portion of the interstellar medium. We don’t really know what it’s made of, but for it to hold galaxies together whilst being completely invisible, we know it has to satisify a few properties:

  1. Dark matter must interact strongly with gravity, but weakly (if at all) with light. It’s similar to how you and I are attracted to the Earth, but not so much to magnets: Dark matter has to be invisible to telescopes just as we are invisible to metal detectors. However, it must still have a strong gravitational pull, which we can infer from how visible matter moves around it. This property rules out most things you can think of (planets, stars, gas and dust, etc).
  2. Dark matter must be stable. We have detected galaxies, both in the very early universe as well as near the present, which are unlikely to form without dark matter. This implies dark matter must have existed for a long time. This property rules out things which decay relatively (free neutrons, tiny black holes, etc)
  3. Dark matter has to be “cold” — physics-speak for slow — and gravity can only clump things together if they are moving slowly relative to each other. For instance, the International Space Station is currently moving at a few hundred meters per second in order to stay in orbit. If we eliminate the station’s orbital velocity (or more formally, its angular momentum), gravity will take over and “clump” it together with the Earth.

The stuff between galaxies

But that’s just the stuff inside galaxies. If the distance between neighboring stars is vast (it takes light more than 4 years to travel from here to Alpha Centauri), the distance between neighboring galaxies is truly cosmic. And between galaxy neighborhoods? incomprehensible. Inside galaxies, the medium and the stars are roughly balanced. Out in the intergalactic wilderness, the wispy medium is nearly all there is.

[Work in progress!]

Just as there’s far more grass on the plains than sheep, and far more water in the seas than fish, the full background medium of the universe actually far, far outweighs the stars! We believe as

This medium contains loose gas, dust, and most mysteriously, dark matter. (more to come)