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This chapter of the video

shows that the bulk of dark matter is not made of planets, brown dwarf stars, or black holes.

One of the earliest theories of dark matter (from the 1970s) was that it is made of known celestial objects such as Jupiter-sized planets, brown dwarf stars, and black holes. These, and other related objects, are known collectively as massive astrophysical compact halo objects (MACHOs) and so the theory that dark matter is made of them is called the MACHO theory of dark matter.

Planets
The main problem with the idea that planets make up the bulk of dark matter is that there would need to be so many of them. For example, there is 39 billion Suns of dark matter in Triangulum within a radius of 4.0 x 10²⁰ m. Jupiter's mass is one thousandth of the mass of the Sun and so we would need more than ten trillion Jupiter-like planets to account for all of this dark matter. This corresponds to thousands of planets for every star within a radius of 4.0 x 10²⁰ m, as in Figure 16. Given that our Solar System has only eight planets, this seems highly unlikely.

Brown Dwarfs and Black Holes
Two other dark matter candidates are brown dwarf stars (also known as brown dwarfs) and black holes, as in Figures 17 and 18. Both have mass but give off so little light that we cannot readily see them using telescopes. In spite of this, physicists can detect their presence via smallscale gravitational lensing experiments. Instead of trying to observe distortions in the images of entire galaxies, they look for distortions in the images of individual stars. These distortions are the temporary brightening of stars caused by dwarf stars, black holes, or other "dark" objects bending nearby light and acting like a converging lens. Physicists have found some unseen mass this way, but not anywhere near enough to account for all dark matter.

A second reason why it is unlikely that the bulk of dark matter is made of black holes relates to the explosions called "supernovas" that accompany their creation. Supernovas occur when a very massive star reaches the end of its lifetime and collapses under gravity. This is followed by an incredibly bright explosion (a supernova) that spews out vast quantities of a wide range of chemical elements, as in Figure 19. If the star's mass prior to the supernova is greater than around 25 times the mass of the Sun, the force of gravity in the collapse is so strong that it results in a black hole.

The elements created in a supernova emit characteristic emission spectra and can be readily detected by physicists. So, the creation of a black hole leaves a highly visible trace. If dark matter were made entirely of black holes, there would be vast quantities of the elements created in a supernova spread throughout the universe. However, current observations indicate that there are nowhere near enough to support the idea that dark matter is composed entirely of black holes.

Visualizing a Black Hole
At various points in this chapter of the video, we show an image of a black hole surrounded by brightly coloured trails, as in Figure 18. They represent electromagnetic radiation emitted by nearby matter as it falls into the black hole, a common occurrence.

Hydrogen Gas
Another dark matter candidate is sparsely distributed hydrogen gas. Hydrogen is the universe's most abundant element and there are vast quantities of it within galaxies, as well as among them. When it is sparsely distributed, it can be challenging to detect.

However, there is strong evidence that hydrogen gas (or anything else made of atoms) does not make up the bulk of dark matter. This evidence comes from the highly successful theory of Big Bang Nucleosynthesis, which allows physicists to estimate the total amount of mass in the universe made up of any type of atom (the baryonic mass). The universe's total baryonic mass is only one fifth of the total mass of dark matter and so it seems that, at best, only a small fraction of dark matter is made of hydrogen gas.

Neutrinos
Yet another theory about dark matter is that it is made of neutrinos. These are tiny, very light subatomic particles that
pass through solid objects as if they were not there, making them very hard to detect. There are vast quantities of neutrinos throughout the universe and so some researchers in the 1980s thought they might make up the bulk of dark matter. Recently, however, physicists have been able to estimate the mass of the neutrino and have found that it is too small to account for the majority of dark matter.