But wait, there’s more! Because dark matter does not interact with radiation (if it did, we could see its light), it can clump together more efficiently than atoms, which tend to heat up and get agitated. The efficient clumping of dark matter allowed it to create dense patches that spurred the formation of the first galaxies. Put dark matter into the models, and they can pretty much explain the structure of the modern universe. Without dark matter, the models fail miserably.
A few researchers, most notably Mordehai Milgrom at the Weizmann Institute in Israel, have perceived a possible flaw in these deductions. Almost all the evidence for dark matter ultimately depends on its gravitational effects. Perhaps the reason the universe looks strange is not because of dark matter, but because the standard theory of gravity is slightly wrong, Milgrom suggests.
The problem with his argument is that no single modification of gravity can explain all the different phenomena outlined above. Making everything fit requires a new theory of gravity, plus additional undiscovered long-range forces, and still requires some exotic dark matter on top of that. There is no escaping the dark truth.
From Darkness, Light
Dark matter is not a hunch, nor is it some kind of scientific sleight of hand. It is an actual, physical thing discovered indirectly but definitively based on multiple independent lines of evidence.
That evidence leaves astronomers in an uncomfortable place, with the vast majority of the cosmos missing. Fortunately, the current situation still leaves a lot more places to investigate. After the LUX experiment completes its run, the affiliated researchers will regroup to build an upgraded version called LUX-ZEPLIN (nerd band name alert), or LZ, which will be 10 times as large and sensitive. In the meantime, a competing team is upgrading a device called the Axion Dark Matter eXperiment (ADMX), which will search for axions, a hypothetical dark particle so slight that it would sail right through LUX undetected.
The ongoing study of dark matter is a precious exercise in “exploring connections” — not just between different fields of science, but also between humans and the cosmos.
It’s possible that both approaches will fail. Then the process of finding dark matter becomes exponentially harder, though not necessarily impossible. Dark matter particles might occasionally destroy each other, creating a visible signal (like the now-discredited Fermi observation). Or they might interact with each other to produce organized structures, such as a thin, dark disk embedded within our galaxy. A dark disk could have an ominous effect, gravitationally destabilizing our solar system every 30 million years or so and setting loose a rain of comets. Such a comet shower could potentially trigger a mass extinction — hence Randall’s link between dark matter and dinosaurs.
A variety of sky surveys, from both space and terrestrial observatories, are seeking dark structures in our galaxy and beyond. Whether or not they confirm Randall’s specific model of dark matter, those surveys will provide a lot more information about how the dark stuff behaves and what it might consist of. They will provide a more accurate cosmic map of all matter, both the seen and the unseen. Above all, Randall says, the ongoing study of dark matter is a precious exercise in “exploring connections” — not just between different fields of science, but also between humans and the cosmos.
As much as it is a source of confusion, dark matter is also a source of illumination. It is all around us. It is almost surely passing through you right now. Our galaxy would not exist without it. We literally owe it our lives.