Dark energy and dark matter make up about 96% of the universe. Baryonic matter, the stuff of stars, planets, you and me, makes up only 4% of the universe. And most of that isn’t actually in stars and planets and us, but between the vast expanse of galaxies. It makes up the intergalactic medium (IGM) and it is notoriously difficult to observe.
Hubble’s Tuning Fork
Edwin Hubble is perhaps most famous for discovering a relationship between the distance of a galaxy and the speed at which a galaxy moves away from us. This relation is now known as Hubble’s Law, and is evidence for the expansion of the universe. But Hubble’s primary interest was in galactic nebulae (what we now just call galaxies).
Primeval Atom
The origin of the universe is often portrayed in popular science as a vast sea of darkness. Centered in this darkness is a bright point of light, which suddenly expands, filling your view with light, fading into a dance of galaxies. Of course this raises all sorts of questions: What did the universe expand into? What triggered the initial explosion? Where did all that matter and energy come from? The problem is, this isn’t how cosmologists see the big bang at all.
Dusty Starburst
A starburst galaxy is a galaxy that’s producing stars at a very high rate. The rate of star production is so high that it would use up the available gas and dust in a time period much shorter than the typical age of a galaxy. We often see starburst galaxies in the midst of a merger or close encounter with other galaxies, which would indicate that starburst periods can be triggered by galactic collisions.
Radio Galaxy
A radio galaxy is a galaxy that emits large amounts of radio waves. They are powered by the galaxy’s supermassive black hole.
Hot and Cold
Part of the evidence we have for dark matter is through its gravitational effect on the motion of stars. The first evidence for dark matter came from motion of stars in our galaxy, which indicated there must be a large quantity of unseen mass in our galaxy. So why is it that when we look for the gravitational effect of dark matter on nearby stars, we don’t see anything? It turns out that tells us something very interesting about the nature dark matter.
Great Attractor
There a region of space about 150 million light years away that is gravitationally attracting the galaxies in its region, including ours. It is known as the great attractor, and we’re not entirely sure what’s there. The problem is it happens to lie in the direction of the zone of avoidance, so our own galaxy is blocking our view.
Zone of Avoidance
Our location within the Milky Way means that our view of the cosmos is somewhat obscured. One way we deal with this is by defining a zone of avoidance.
Push Me, Pull You
If the universe is expanding, how is it possible that galaxies can collide with each other? It turns out that not only are colliding galaxies possible in an expanding universe, the fact that they occur is due in part to dark matter and dark energy.