The universe doesn’t behave the way we think it should.
We have a wealth of evidence that the way galaxies form and move, the way gravity affects light, and the very expansion of the universe itself, do not behave the way we would expect from the matter that we can see.
Such observations are the reason for physicists’ claims that 95% of the universe is composed of things other than “normal” matter, i.e the stuff which makes up the world that humans see and inhabit.
Being logical creatures, physicists invented “dark matter”, to wit “extra stuff floating around in the universe that we can’t see”. This is perhaps not the most imaginative name that could have been chosen…
When some new observations (the history of this is often over-simplified and focussed on the work behind the 2011 Nobel Prize, which isn’t quite correct) suggested a second invisible thing was needed in the universe, one which behaves differently from the dark matter, a new name was needed. Hence, “dark energy” was born.
Note that I am intentionally not going into much detail here; this post is mostly just to set the scene that we don’t know what 95% of the universe is made of, and that we explain our current observations of the universe using two different invisible things: dark matter and dark energy. I will naturally go into more detail on most of the topics alluded to here in future blog posts, as I introduce things that are currently being worked on.
I will just mention that the current model of cosmology comprises the simplest possible possibilities for the dark sector: cold dark matter (which doesn’t move fast and only talks to itself and other things via gravity) and a cosmological constant (as the dark energy). This latter has been famously (and melodramatically) called Einstein’s “biggest mistake” or “greatest blunder”.
However, the science is not settled yet. No dark matter particle has yet been found in nature, and we only have gravitational evidence for the dark sector. This latter is the reason why one possible alternative to the dark sector is that we are not using the correct theory of gravity: perhaps Einstein was wrong? I must admit to having a soft spot for this possibility.
Maybe the current simplest model of a cosmological constant plus cold dark matter will ultimately win and a dark matter particle will be found. Maybe the answer will turn out to be something that no-one has thought of yet. Let us find out together!
Dark Matters 