It’s not revealed in the article for this particular case, but the usual way of determining the dark matter content works by estimating the total mass or mass distribution, and subtracting the visible mass of stars.
The total mass can be estimated from gravitational lensing, and for rotating galaxies the distribution of mass within the galaxy can be estimated using the speed of rotation of stars at different distances from the center.
"Rotating" meaning sighted at an angle so you can use Doppler shifts of familiar spectra to figure out the tangential velocity of parts of the galaxy. In this case they used what they called "gas", meaning I suppose interstellar plasma, as the reference for the Doppler-shifted waves. They could check emission lines from the plasma, or reflections off it, or gaps in transmitted light representing absorption of that light by the plasma.
The total mass can be estimated from gravitational lensing, and for rotating galaxies the distribution of mass within the galaxy can be estimated using the speed of rotation of stars at different distances from the center.