If it is invisible, how do they see it?

Astronomers have discovered an invisible galaxy that could be the first of many that will help unravel one of the universe's greatest mysteries.
The object appears to be made mostly of "dark matter," material of an unknown nature that can't be seen.
Theorists have long said most of the universe is made of dark matter. Its presence is required to explain the extra gravitational force that is observed to hold regular galaxies together and that also binds large clusters of galaxies.
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"From the speed [the "invisible galaxy"] is spinning, we realized that VIRGOHI21 was a thousand times more massive than could be accounted for by the observed hydrogen atoms alone," Minchin said. "If it were an ordinary galaxy, then it should be quite bright and would be visible with a good amateur telescope."
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Dark matter makes up about 23 percent of the universe's mass-energy budget. Normal matter, the stuff of stars, planets and people, contributes just 4 percent. The rest of the universe is driven by an even more mysterious thing called dark energy.