Why don't all galaxies have identical amounts of dark matter? This question holds significant implications for our understanding of the universe. Two fundamental assumptions underpin our grasp of cosmology: first, the physical laws governing the universe are universally applicable; second, the universe originated with uniform properties. While observational data from various celestial objects support these assumptions, we cannot assert their absolute truth.

Despite the universe adhering to the same laws of physics and originating from similar conditions, the outcomes we observe today can differ widely. The universe is intricate, filled with a mix of ordinary matter that forms stars, dark matter that exerts gravitational influence, and nearly 14 billion years of evolution. While there may be approximately 2 trillion galaxies in the observable universe, they are not all identical.

Visualize the universe shortly after the Big Bang: hot, dense, and nearly uniform. Particles and radiation exist in remarkably similar quantities, with variations barely exceeding 0.003%. Although matter experiences gravitational attraction, the radiation prevents significant growth in denser regions.

Over time, this initial state evolves as the universe expands and cools, leading to a decrease in density and a drop in radiation energy, which in turn allows matter to collapse gravitationally. Initial density variations gain mass, leading to the formation of stars and galaxies.

In this burgeoning universe, galaxies emerge with diverse masses. The smallest may have a few hundred thousand solar masses, while the largest can reach trillions. Initially, all galaxies start with a similar dark matter to normal matter ratio of approximately 5-to-1.

However, as galaxies form stars, this ratio begins to shift. Only normal matter can form stars, interacting through various forces beyond gravity. Dark matter, though it also experiences gravity, does not engage with other fundamental forces.

The formation of stars triggers several key processes: 1. New stars emit significant radiation, particularly ultraviolet, which interacts with surrounding normal matter but not dark matter. 2. Stellar winds from young stars energize normal matter around them. 3. Supernovae from massive stars release vast energy, absorbed by normal matter but not by dark matter.

While normal matter absorbs this energy, dark matter remains unchanged, only responding to gravitational alterations caused by the shifting distribution of normal matter.

For massive galaxies, the abundance of both normal and dark matter allows them to retain their normal matter despite energetic events. However, smaller galaxies that have experienced significant star formation may lose much of their normal matter, leaving predominantly dark matter.

Low-mass galaxies, particularly dwarf galaxies, often exhibit high ratios of dark matter to normal matter. Some may have ratios as extreme as 600-to-1, reflecting their diminished ordinary matter.

Interactions and collisions between larger galaxies can further disrupt the balance of normal and dark matter. High-speed encounters with intergalactic gas can trigger star formation and strip gas from the traveling galaxy. Mergers can eject normal matter, while tidal forces can pull internal gas out of one or both galaxies.

These processes can lead to a higher ratio of dark matter to normal matter, potentially resulting in galaxies with little to no dark matter. When normal matter is stripped from a galaxy, it can self-gravitate, creating a dwarf galaxy that may contain less or even no dark matter.

The intriguing possibility of discovering a galaxy devoid of dark matter could further substantiate the existence of dark matter itself, as it would demonstrate that two types of matter can behave differently.

The critical question is: where are these dark matter-free galaxies located? They likely exist in environments with larger galaxies and may not last long, as galactic interactions and mergers have primarily occurred in the universe's distant past. Once a large galaxy captures these dark matter-free galaxies, they may no longer exist.

Finding these galaxies is challenging due to their faintness and limited number of stars. It's unlikely to encounter a Milky Way-like galaxy without dark matter; instead, only small dwarf galaxies present this possibility. If most of these galaxies formed 8 to 9 billion years ago, few may remain today.

Nevertheless, advancements in astronomical techniques may soon enable the identification of galaxies without dark matter. NGC 1052-DF2 and NGC 1052-DF4 are potential candidates, but further observations are required to confirm their properties and distance.

Ultimately, the presence or absence of dark matter in one or two galaxies may not provide conclusive evidence. The vast number of dwarf galaxies, many currently beyond our observational limits, could hold the key to understanding dark matter's nature. As we explore these distant galaxies, we might finally uncover this elusive population.

If dark matter is a reality, it must be separable from normal matter. We have identified dark matter-rich galaxies and isolated intergalactic plasma, but dark matter-free galaxies could be just around the corner, fueling excitement in the astronomical community.

Starts With A Bang is now on Forbes, and republished on Medium thanks to our Patreon supporters. Ethan has authored two books, Beyond The Galaxy, and Treknology: The Science of Star Trek from Tricorders to Warp Drive.