Dark matter and dark energy are two of the most profound mysteries in contemporary physics. Despite the evidence supporting them, we still cannot directly observe them or confirm they exist. To show the scale of the problem, just 5 percent of the observable universe consists of regular matter, the remaining 95 percent is thought to be this dark matter and dark energy. In this interview with Hongsheng Zhao, the IAI’s Max Rogers, enquires about Zhao’s theory that dark matter and dark energy are really one thing: Dark Fluid. This theory may have the potential to help us understand quantum gravity and the origin of mass.

Where does the story of dark matter and dark energy begin?

Up until the 1970s, particle physics experienced rapid advancement through the continuous discovery of new particles. Once this phase of discovery reached its limit, physicists constructed a model known as the standard model, which neatly incorporates all the particles that had been discovered up to that point—almost all of them. But there were still several missing particles, notably the mass-giving particle called the Higgs Boson, as well as some cosmological issues. This is where dark matter and dark energy enter the picture.

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Dark matter and dark energy were introduced to explain important observations that the standard model failed to account for

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Why are they called ‘Dark’ and why were they introduced?

Particle physicists postulated the existence of additional particles, coined dark particles (i.e., dark matter), for theoretical completeness. What distinguishes dark matter from normal matter is its lack of interaction with the electromagnetic force: dark particles neither absorb, reflect, nor emit light. Consequently, they are particles which astronomers cannot directly observe in light; we can gather indirect evidence, only through their gravity, e.g., their gravitational bending of background stellar light.

Dark matter and dark energy were introduced to explain important observations that the standard model failed to account for. The foremost among these are the centrifugal acceleration in rotating galaxies and the accelerated expansion of the universe. Galaxies rotate at a disproportionately fast rate compared to the amount of gravity from ordinary matter present, prompting the introduction of dark matter to balance this excessive rotational centrifugal force. In other words, there must be additional gravitational effects at play that are invisible to us, explaining the rotational patterns of galaxies. Additionally, cosmological observations reveal that the universe's expansion is accelerating over time. Dark energy was posited to address this phenomenon. Now, we are finding more evidence on even larger scales - larger than galaxies - supporting the need for new physics with effects like both dark matter and dark energy.

We often hear that approximately only 5 percent of the matter in the universe is ordinary matter, with the other 95 percent being composed of dark matter and dark energy. How do we know this relative proportion with such precision?

Picture galaxies springing out of the Big Bang into a marathon with a huge spread in speed, the gravitational attraction of dark matter would make the relative speed between galaxies reduce with time, until an epoch when their speed gap widens again as galaxies “feel” new energy.  The specific ratio of matter and energies is parametrised to explain why we are seeing such an epoch in galaxies now.

What distinguishes dark energy from dark matter, and why are they frequently grouped together?