A galaxy is like a giant merry-go-round: stars and gas whirl around the center, and their speeds reveal how much stuff is on the ride. If we only saw the luminous matter, the outer parts would spin slower than observations show. So there must be invisible mass, dark matter, that holds the galaxy together.
Astronomers took 20 galaxies from the MIGHTEE survey and built a detailed rotation map for each. To figure out how much ordinary matter (stars and gas) a galaxy contains, you need to know how much light each star gives off. Usually, one average number is used for the whole galaxy, but here they applied a smarter approach for the first time: they estimated stellar brightness separately in different regions. It's like weighing each slice of a cake instead of the whole cake at once. This method allowed them to separate the stars' contribution from the dark matter's more precisely.
The result: when we account for the fact that different parts of a galaxy shine differently, dark matter models (the so-called Navarro–Frenk–White profiles) fit the data much better. This means future dark matter studies should look at individual regions of a galaxy, not just the whole, or else we might draw wrong conclusions about the hidden side of the universe.
The same principle works in everyday life: if you want to know how much sugar is in a fruit compote, tasting one spoonful from the top isn't enough: you need to stir and assess the whole volume. Astronomers now know how to 'stir' the light of galaxies, bringing us closer to solving the dark matter puzzle.
Galaxy rotation speeds are measured by the shift in radio waves from hydrogen clouds, just like estimating a car's speed from the change in pitch of its horn.