Scientists have finally been able to accurately calculate the weight of the Milky Way, overcoming the difficult hurdle of measuring dark matter, the European Space Agency (ESA) announced Thursday.

After years of struggling to estimate the size of our galaxy, astronomers with NASA and the ESA used data from the Hubble Space Telescope and the ESA’s Gaia mission to determine the Milky Way weighs about 1.5 trillion solar masses within a radius of 129,000 light years from the center.

Because dark matter makes up about 90 percent of the galaxy, estimates of the Milky Way’s weight have differed widely in the past. Previous measurements ranged from 500 billion to 3 trillion times the mass of the Sun.

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"We just can't detect dark matter directly," Laura Watkins, of the European Southern Observatory in Germany who led the team’s analysis, said in a statement. "That's what leads to the present uncertainty in the Milky Way's mass – you can't measure accurately what you can't see."

Because dark matter is so difficult to calculate, Watkins and her team measured the velocities of dense star clusters, called globular clusters, that orbit the galaxy’s spiral disc.

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They used data from Gaia, the ESA’s space observatory, to measure globular clusters as far as 65,000 light-years away from Earth and data from the Hubble Space Telescope — a project shared by NASA and the ESA — to measure globular clusters as far as 130,000 light-years away from Earth.

"We were lucky to have such a great combination of data," Roeland P. van der Marel of the U.S. Space Telescope Science Institute said.

"By combining Gaia's measurements of 34 globular clusters with measurements of 12 more distant clusters from Hubble, we could pin down the Milky Way's mass in a way that would be impossible without these two space telescopes," van der Marel added.