The other ‘stuff’ in our galaxy… what is it?

So what is this mysterious concept called dark matter? Physicists tell us that matter comprises a very insignificant portion of the universe, which leads to the question- what makes up the rest of the stuff?

Physical matter is believed to comprise less than 4% in the intergalactic.. goo? Source: ASP

Let’s start with dark matter- what is it? How do we observe it? It turns out that our own galaxy is greatly affected by dark matter. The Milky Way has obtained a noticeable warp in its shape, which was first observed by astronomers analyzing the oscillations of the hydrogen gas disk. It was noted that oscillations occur such that at a distance of roughly 75,000 light years away from the center, the disk has shifted 7,500 light years out of the plane. The weirdest thing is that these oscillations are actually three different oscillations, which have been characterized by astrophysicists to be “64 octaves below middle C.” Where does this warp come from? You guessed it- dark matter. The neighboring Large Magellanic Cloud (LMC) only occupies less than 2% of the matter of the Milky Way, but its movement (1.5 billion year orbit around the Milky Way) “acts as a gong vibrating in slow motion.” Strange indeed that dark matter is only observed by its gravitational presence!

Warp in Milky Way caused by LMC/SMC. Source: Scientific American

Dark energy is observed in a different way- via cosmological expansion. The Standard Model initially predicted that after the Big Bang, expansion should decelerate. However, supernova explosion data looks dimmer for a given redshift, which means that the explosion has moved further away than initially predicted. Translation: universal expansion is accelerating. But is this true in all areas? Uncertain. Perhaps the universe is inhomogeneous and expansion doesn’t occur uniformly! This would explain the inhomogeneity in the CMB, discussed in a previous post. It also means that we are not necessarily in the “center of the universe” and that expansion is driven by changes in dark energy density.

Further information on dark matter that might be interesting- be sure to check out the video! That was pretty helpful to understand the warp:

Dark matter


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