Caption: Galaxy rotation curve for M33 (AKA the Triangulum Galaxy) superimposed on a to-scale image of M33 (one supposes).
Features:
It is 0.73--0.94 Mpc away and has radial velocity -179(3) km/s.
It is type SA(s)cd.
We call this gas of exotic particles, the dark matter.
This null result has led some to argue that there is NO dark matter and that the law of gravity needs to be radically revised.
This counter theory has some supporters, but it is still a minority view.
Maybe we live in the worst of all possible worlds where there is dark matter AND we have to radically revise the law of gravity.