The dinosaur extinction 66m years ago was most likely due to a comet or huge asteroid crashing into Earth, but knowing that asteroids don’t actually impact our planet very often, could this really be the entire story? Many scientists are now wondering whether some sort of cosmological event could have boosted the number of comets at the time, making such a collision more plausible.

In a recent book, American cosmologist Lisa Randall points out that a huge disk of ‘dark matter’ – a type of invisible matter that is five times more common than ‘normal’ matter – could have been the cause. When sweeping past our solar system, such a disk would cause a tiny distress in space, cumulating to a flicker in the gravitational force that can push comets out of the solar system’s Kuiper belt, or the Oort cloud just outside and send them towards the Earth.

But can this theory be trusted? And are there any other cosmological events that could shed light into this matter? Mounting astrophysical and cosmological evidence indicates that there is a lot more dark matter in our galaxy than normal matter. Although it is invisible, we know of its existence because of the gravitational pull that’s affecting objects surrounding it.

Being dark simply means that it does not diffuse or absorb light, which makes it tricky to see. Most cosmologists believe this matter, which is after all part of our galaxies and galaxy clusters, moves slowly, and is ‘cold’ (because hasty particles are hot.)

Randall claims that there’s an entire disk of dark matter in our own galaxy. For it to have an effect on us it would need to be practically aligned with the visible disk of the Milky Way so that the solar system will move back and forth around it as it traverses the galactic center..

To get to the bottom of this, we have to make dark matter weirder than it already is. Randall considers that there is more than one type of dark matter in the form of a ‘contamination’, which she says could consist of 5-10% of the total dark matter. This particular kind of dark matter differs because it can interact with itself just like normal matter does.


While the majority of dark matter can flow through itself without pausing, this special so-called “dissipative” dark matter can cut-off its movement and thereby form a galactic disk, like normal matter does. However, Randall admits in her research papers that there are insecurities regarding this dark matter forming an actual disk.

Randall is a renowned cosmologist, so her hypothesis is certainly credible. Unless the model doesn’t debunk future observations, Randall believes there is a slight possibility in which her scenario would actually result in an increase in comets and asteroids.

So is there any proof of this in the geological or paleontological record? While the issue is still being discussed, there is no conclusive evidence that extinctions have happened cyclically. Randall’s team of researchers believes the boost in comets may happen every 35m years or so, and that it could coincide with the mass extinctions.

So, despite a number of unknown variables, is a dark disk the best cosmological explanation for mass extinctions, or rather the ‘spatial boulder’ which presumably impacted Earth?


Another theory that has been put forward is that the sun has a companion star called Nemesis, a faint red/brown dwarf star orbiting the sun at a distance of approximately 1.5 light years. Every 25m years or so, it makes a step closer to the sun, which could result in enhanced comet activity due to its gravitational pull. This theory is not unlikely, since the majority of stars belong to systems with multiple stars, and judging by the latest discoveries of unknown planetary masses orbiting at the edge of our solar system, as well as the strange ‘Dimming Star’ found relatively close to our planet, Nemesis might prove to be more than a myth after all..

Randall’s scenario is lacking palpable evidence, and although her study offers a more detailed view than what is presented above, the fact that there is no real evidence of periodicity in mass extinctions and crater formation on Earth is a problem. However, the proposal is certainly not impossible and should be taken into account when making observations. Summing up, it reminds us that basic physics and cosmology are fundamental aspects of nature that may even impact the flow of life on Earth.