October 31st is celebrated as Dark Matter Day (amongst other things). Dark matter, as its name suggests, cannot be seen and its presence can only be inferred from the gravitational affect it has on objects we can see. First invoked to explain why galaxy clusters didn’t fly apart, compelling evidence was obtained by Vera Rubin and Kent Ford in the 1970s, who showed that most stars in the spiral galaxies they studied orbited at roughly the same speed. It had been expected that stars further from the centre of the galaxy would be orbiting more slowly, as is the case for planets orbiting the Sun. The fact that this was not the case indicated there must be a large amount of unseen matter in a large galactic halo. Radio astronomical observations of the 21cm HI (neutral hydrogen) line have confirmed this. One such example is shown above, from ASKAP WALLABY pilot survey data, where the rotational speed of the gas (on the y-axis) is plotted as a function of distance from the centre of the galaxy (on the x-axis). It is now believed that roughly 25% of the Universe is made of dark matter. Less than 5% is visible, detectable matter like stars and planets, and the bulk of the Universe is the even more mysterious Dark Energy.