Can observations of galaxies be explained without dark matter?

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After almost a century and a half, we are still completely in the dark about dark matter. What was once thought to be matter too dim for telescopes is now considered to be a different type of matter altogether. Two independent researchers, Alan Sipols and Alex Pavlovich study galaxies in their spare time. In their most recently published research, they computed maps of mass in 214 different galaxies and compared these to brightness maps. In 62% of galaxies, they found no discrepancy between brightness and mass, and found that in the rest the excess of ‘unseen mass’ was explicable by ordinary factors, with no need for dark matter.

Of all the sciences, it is perhaps physics which continues to provide us with the greatest number of mysteries. String theory, quantum gravity, matter-antimatter asymmetry, a theory of everything: each represents another murky, unexplainable depth to the Universe.

Perhaps one of the most fascinating and delightfully simple of these unsolved problems is this: lots of galaxies seem to contain too little mass compared to how fast they spin. Many galaxies should fly apart simply due to the speed at which they spin: conventional estimates of their mass indicate that there simply should not be enough of a gravitational pull to hold them together.

Lord Kelvin was perhaps the first to point this out, when he concluded an 1884 talk by suggesting that “many of our stars, perhaps a great majority of them, may be dark bodies”. Could these galaxies contain matter which we cannot see?

NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration

The idea caught on, and what became named ‘matière obscure’ in 1906 by Henri Poincaré, and was known to Swiss astronomer Fritz Zwicky as ‘dunkle Materie’, is called in English ‘dark matter’. Over decades, the concept of dark matter has become a popular explanatory patch for several unresolved problems in astrophysics and cosmology, including the well-known problem of galactic rotation.

The dark matter paradigm

The generally accepted view by physicists is that dark matter is some sort of a mysterious ‘non-baryonic’ material composed of strange particles which can interact with regular ‘baryonic’ matter via gravity. This hypothesised exotic substance devoid of electromagnetic reality is thought to represent up to 85% of all the matter in the universe and clump into colossal sphere-shaped halos enveloping entire galaxies and gravitationally controlling their rotation.

“Does the observed galactic rotation match the observed galactic light?”

However, a few researchers think that the dark matter hypothesis is incorrect. Amongst them are Alan Sipols and Alex Pavlovich. Both are independent researchers, working alongside their day jobs: Alan is an entrepreneur, and Alex has a career in investment banking. In collaboration, they moonlight as astrophysicists, and take a different view to the institutionalised consensus. Their work, analysing astrophysical datasets through the prism of novel models, leads them to the conclusion that there really is no need to believe in dark matter.

Shining a light on dark matter

These researchers are not by any means the first to call into question the existence of dark matter. Some early theorists proposed that in certain circumstances gravitational force could be stronger than traditionally thought, and so less mass is required than we think to hold these galaxies together. Others suggest that the unseen matter is not a new type of matter, but just ‘dim’ matter such as brown dwarfs, black holes, and cold hydrogen gas.

Whilst both ideas are attractively simple, the first is in essence a fanciful modification of the laws of physics, and the second, although likely part of the explanation, never fully addressed the problem given the amount of ‘dim’ regular matter in galaxies required to account for the gravitational effects attributed to dark matter halos.

What sets Sipols and Pavlovich’s approach apart from these attempts to dismiss dark matter is that their models remain completely rooted in classical physics whilst requiring a lower amount of mass that is in good agreement with the amount of observed light.

New approach to dark matter analysis

At the heart of their research is the question: does the observed galactic rotation match the observed galactic light? More specifically, they are interested in computing how the rotation-compliant mass must be distributed across each galaxy and how such distribution of mass compares with the observed electromagnetic radiation.

Typically, a distribution of mass in a galaxy is inferred from the distribution of brightness across the galaxy, based on a certain stellar mass-to-light multiple which is arbitrarily set to be the same throughout a galactic disk. This inferred mass distribution is then compared to the rotation pattern, revealing the need for dark matter preconceived as a template spherical halo.

Sipols and Pavlovich take a different approach, starting from the other end and avoiding conventional, but unproven assumptions of the radially uniform stellar mass-to-light ratio in galaxies, and of the spherical shape of a dark matter halo. With the aim of comparing mass to light as independent data series, they derive the distribution of galactic mass solely from the observed rotation patterns.

To compute galactic mass density distribution, the researchers employ the Disk Mass Density Model (DM2) in which a galaxy is treated as a rotating disk. They use observed galaxy rotation data as the sole input into their model to compute how mass should be distributed across the galaxy to match its observed rotation.

A galaxy’s computed density profile is then compared with its observed luminosity profile. For each area of the galaxy, the aim is to check whether the combination of the two metrics – the density of the local mass and the brightness of the local radiation – satisfies the well-known statistical relation between an individual star’s mass and luminosity.

If there is good agreement between the local density and the local brightness – that the mass at each point in the galaxy could produce that amount of light – then there is no need for non-baryonic dark matter to be part of local mass which can be fully attributed to regular luminous matter.

“Galaxies should exhibit a decrease in average star mass as one moves away from the galactic centre towards the outskirts.”

214 galaxies without dark matter

In a recent paper published in Galaxies journal, Sipols and Pavlovich apply this analytical framework to a diverse sample of 214 galaxies. The researchers believe it is the largest mass-luminosity study of galaxies to date.

They show that for 62% of galaxies the combination of local mass density and local brightness can be fully explained, for all areas of galaxies where both rotation and brightness data are available, by fitting a ‘characteristic star’ with a matching ratio of mass and luminosity.

In the remaining 82 galaxies, the average excess density is so low – with a mean of about 14 solar masses per square parsec – that this can be attributed to red dwarf stars, stellar remnants, substellar and non-stellar ‘low-brightness’ baryonic matter that is not yet resolvable with current telescope sensitivity, as well as to the notorious ambiguity of astronomical data such as distances to galaxies.

ESA/Hubble & NASA, A. Riess et al.; Acknowledgment: M. Zama

An intriguing hypothesis

In their paper, Sipols and Pavlovich conclude that galaxies should exhibit a decrease in average star mass as one moves away from the galactic centre towards the outskirts. As per this trend, stellar populations at galactic periphery should be dominated by faint low-mass stars.

Could there be a reason behind such an uneven distribution of star sizes? The authors suggest that a characteristic star mass must correlate with the availability and density of the star-forming medium. They point at the higher concentration of massive O-type stars in the centre of the Milky Way and to discoveries of extended faint stellar populations in other galaxies.

This proposal may not be immediately popular in the corridors of astronomy departments, but Sipols and Pavlovich point out that the hypothesis they advance is mundane when compared to the idea of a new kind matter, and is directly verifiable with the next generation of high-resolution telescopes. Finding it easier to believe in conceptually simple explanations of observed phenomena and hinting at futile experiments to detect dark matter particles, they rule out dark matter in galaxies and playfully compare the elusive substance to Confucius’ black cat: “It is hard to find in a dark room, especially if there is no cat”. With over 200 galaxies of different size and type shown to be dark matter free, maybe there is indeed no cat in the room. Time will tell.

There is other evidence for dark matter, including observations of gravitational lensing. Are effects like this also explicable without dark matter?

There are lensing studies which reveal problems with dark matter models at supra-galactic scales, but this is not the part we have investigated so far. Our focus has been on galaxy-level dynamics which is arguably the most data-rich and the least ambiguous pillar of the dark matter hypothesis. So, to answer “yes” without examining in detail would be jumping the gun. Having said that, dark matter has grown into a cosmic Jack of all trades and a master of none: a cure for all dilemmas, if only it was real. We expect that this convenient fix-it-all shortcut for various analytical and data problems will eventually get replaced with different, context-specific real solutions.




Research Objectives

Alan Sipols and Alex Pavlovich study galaxies, explaining galactic phenomena without the need for dark matter.


Alan Sipols is an independent researcher and entrepreneur. With a background in complex systems modelling, he pursues research goals in multiple domains of science. In astrophysics, he collaborates with Alex Pavlovich to study galaxies.

Alex Pavlovich combines a passion for science with a career in investment banking. Interested in independently auditing the non-baryonic dark matter hypothesis, he has mastered astrophysics and engaged in galaxy research projects together with Alan Sipols.


Alan Sipols

Alex Pavlovich

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