All the ‘normal’, visible matter we can see in the Universe – planets, stars<\/a>, galaxies<\/a> etc. – accounts for just 5% of its total content.<\/p> The rest is composed of dark energy<\/a> – the name given to the force that appears to be accelerating the rate of the expansion of the Universe<\/a> – and dark matter<\/a> – which is known to exist, but which cannot be observed directly.<\/p> So the question is this: if dark matter accounts for a vast amount of the known matter in our Universe, why do some galaxies appear to contain almost none of it?<\/p> What does it mean to find a galaxy without dark matter, or with very little of it?<\/p> This is the question asked by Pavel Mancera Pi\u00f1a<\/a>, a postdoctoral researcher at Leiden Observatory<\/a> in the Netherlands, who specialises in in dark matter.<\/p> We spoke to Pi\u00f1a to find out more.<\/p> We have evidence suggesting that there should be a certain amount of mass in the Universe, but this mass cannot be observed directly.<\/p> Even in early works from the 1920s, astronomers knew that in order to explain how fast some galaxies move there needed to be more mass than we could account for.<\/p> An astronomer called Fritz Zwicky came up with the name \u2018dark matter\u2019, but essentially the name only reflects that we don\u2019t really know what it is.<\/p> Most of the community \u2013 not all \u2013 believes that dark matter is made of particles that don\u2019t interact with electromagnetic radiation, instead only interacting with normal matter through gravity<\/a>.<\/p> Essentially, it doesn\u2019t absorb or emit light (and heat). We can only infer that it\u2019s there due to gravitational effects.<\/p> The problem is that although we have built experiments like the Large Hadron Collider, we haven\u2019t been able to detect dark matter particles, so we don\u2019t know what their exact properties are.<\/p> By looking at how fast galaxies rotate. We know the rotation of a galaxy is a consequence of its total mass, so if we can observe how fast a galaxy rotates, then we know its total mass.<\/p> We can compare the total mass measured from how fast the galaxy rotates with the mass of stars and gas [in the galaxy], which we can observe directly from telescopes.<\/p> We can then look at the mismatch between the two to infer how much dark matter there is in the galaxy.<\/p> There is typically a very strong mismatch, suggesting that there is a lot of mass that we don\u2019t see.<\/p> Our team was looking at a galaxy that was very peculiar because it\u2019s classified as an \u2018ultra-diffuse\u2019 galaxy.<\/p>![\"Pavel](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2023\/04\/Pavel-Mancera-Pina-0120a2a.jpg\")
<\/figure>What is dark matter and why is it called \u2018dark\u2019?<\/strong><\/b><\/h3>
What might dark matter be made of?<\/b><\/h3>
![\"Galaxy](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2019\/03\/Bullet-b78c4c6.jpg\" "\\"\\"\/")
How do you look for dark matter?<\/strong><\/b><\/h3>
What do your investigations show?<\/strong><\/b><\/h3>
![\"Ultra-diffuse](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2022\/05\/09.potw2217a-be57476-e1682418088576.jpg\" "\\"\\"\/")