The Universe and the Dark Fluid

© 2025 Kieron Conway - All rights reserved.

There are two fundamental problems in modern physics, which are;

Problem 1:

Spiral galaxies don't have enough gravity to stop their arms from spinning off into space. Something is clearly producing the additional gravitational attraction that keeps spiral galaxies from spinning apart. Whatever it is, it can't be detected, so it's called 'dark-matter'.

Problem 2:

The expansion of the universe is not slowing down as expected, it's actually speeding up. Something is clearly causing the universe's expansion to speed up and we can't see what it is, so the term 'dark-energy' was used to explain the phenomenon.

The fact that both terms use the word 'dark' implies that we haven't a clue what they are.

Enter the Dark-Fluid!

A current, speculative theory suggests that dark-matter and dark-energy are two different manifestations of a single 'dark-fluid,' with their behaviour depending on whether baryonic matter is present or not.

Far from Baryonic Matter

A more up-to-date description of dark-energy, based on observations of its behaviour, is that it acts like a fluid with a large negative pressure that interacts with nothing.

I know this might sound far-fetched, but bear with me. This fluid is the same in all directions; in other words, it's isotropic, a word used throughout our journey to describe the fact that the universe has no preferred direction and appears the same when viewed from anywhere, in all directions.

If we treat dark-energy as a perfect fluid, it can be characterised by just two parameters: density and pressure and they are the same wherever you look in the cosmos. Bizarrely, the density of dark-energy doesn't change, whereas the overall density of the universe decreases as it expands.

Normally, a fluid, or any form of medium, has a positive pressure and to compress such a medium requires effort or energy to increase its density. Gravity, for example, can compress a gas with positive pressure, and we have many examples of gravitational compression, not least in the formation and life-histories of stars.

If dark-energy has a negative pressure, it can't be compressed, and gravity actually expands a perfect fluid with negative pressure.

So, what we have is a repulsive force at work created by gravity acting on the dark-fluid that we call dark-energy. What's more, and this is the really bizarre bit, when gravity does work to expand dark-energy, it actually creates more dark-energy and keeps its density constant.

This is quite a challenging concept: the resulting repulsive force continuously reduces the overall density of the universe while keeping the density of dark-energy unchanged, and this effect accelerates the expansion of the universe.

This is the basis of the 'dark-fluid' theory, which seeks to explain both dark-energy and dark-matter as different manifestations of the same underlying substance.

In Regions containing Baryonic Matter

In the presence of baryonic matter (within a galaxy, for example) the dark-fluid concentrates around the matter and draws in more of itself and its density increases, but only around the baryonic matter.

In general relativity, gravity depends on any form of energy density as well as mass. So, the increase in dark-energy's density around the mass increases the local gravitational force. Consequently, the total gravity in a galaxy is made up from baryonic matter and the localised increased density of dark-energy.

Wrapping it all up:

In regions of concentrated baryonic matter, the dark-fluid is pulled into the region resulting in an increase in the density of dark-energy, which behaves like dark-matter, explaining the missing gravity in spiral galaxies.

In the vast voids of space, far from major gravitational sources, the dark-fluid behaves like a fluid with negative pressure, or like dark-energy, maintaining a constant density as the overall density of the universe decreases and the rate of the universe's expansion increases.

The dark fluid is a highly speculative theory and not part of mainstream cosmology, but one has to admit that it is a very appealing concept.

To learn more

Type 'dark fluid theory' into Google and pick up the Wikipedia link to learn more. It's a fascinating theory.

But Wait - January 2026 Update on Dark energy

Something may be stirring in the astro-physics community. Recent research, unverified at present, indicates that the density of dark energy may not be constant and may be decreasing! To learn more, have a look at the blog article from 8th January 2026, titled: "What is Dark Energy up to?"

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