The evolution of life is a 13.8 billion-year-long chain of events, from the Big Bang through the creation of chemical elements, galaxies, planets and the first self-replicating organisms. But without dark matter's influence, says an astrophysicist, it's hard to see how anything as complex as life could have emerged.
James Bullock, a professor of physics and astronomy at the University of California, writes on his blog that the Big Bang could have been a Big Flop. Our Universe started out with just hydrogen and helium, which by themselves aren't the building blocks for molecules, worlds and certainly nothing as complex as life.
Thankfully, Bullock notes, our universe was also born with dark matter, which gave the early Universe the extra gravitational tug it needed to make something of itself:
Dark matter plays the role of Creator: its gravity is pulling sections of the Universe to buckle back on itself, forming galaxies along the way. Dark energy is doing just the opposite. It's fighting the collapse by propelling the universe to expand at an ever-faster rate. Luckily for us, dark matter has been winning for most of cosmic time, particularly in the all-important early stages. Our Galaxy, the Milky Way, would have never collapsed out of the expanding rush of the Big Bang without the aid of dark matter's pull. That means no Sun, no Earth, and no you.
About 14 billion years ago, when that soup of hydrogen, helium, and dark matter emerged from the Big Bang, everything was expanding. This isn't the best situation for building complexity. No prokaryote is going to spontaneously emerge in a Universe consisting of hydrogen atoms flying away from each other in an expanding horde.
But not every part of the Universe kept expanding….There were tiny irregularities — 0.001% in density — that began to grow over time because of gravity. Areas with more matter attracted even more over time. The dark matter played a key role: it provided extra mass and made structure grow much faster than it would have otherwise. The dark matter also remained much clumpier in the beginning than the normal matter for another reason: it doesn't interact with light. Early on, the blindingly bright ambient photons left over from the Big Bang scattered off of protons, smoothing out the distribution. This process (called Silk Damping) took an already smooth distribution of normal matter and made it even smoother. Light can't scatter off of dark matter, so the dark matter remained relatively clumpy on the length scales that would eventually grow into galaxies.
Had there been no dark matter in the beginning, there would have been a much lower level of primordial structure and much less gravity to make those tiny imperfections grow. The resulting universe today would be unrecognizable. Virtually nothing akin to the galaxies we know would exist.
But as we discovered 15 years ago, dark matter is losing its gravitational tug-of-war with dark energy. The expansion of the Universe is speeding up and as it expands, the density of matter decreases while the amount of dark energy remains constant. Eventually, the stars will burn out, leaving the cosmos dark and empty.
Before that happens, however, Bullock says that complexity will have one last grand hurrah. Fresh fuel for star formation will stop falling into galaxies and no new stars will be able to form, which means the Universe will be flush with heavy atoms and complex molecules. "It will be a last triumphant opportunity for our Universe to produce life," he says "and maybe even a few critters to look up at the stars and wonder how they came to be."