A huge, unknown object punched a giant hole in the Milky Way galaxy, and astronomers have no idea what it was.
At a conference of the American Physical Society in Denver on April 15, Harvard-Smithsonian researcher Ana Bonaca presented data from the Gaia space observatory operated by the European Space Agency (ESA) that she said suggests “a dense bullet of something” invisible to our telescopes shot holes in the galaxy.
The ESA launched the observatory in 2013 to chart a 3D map of the billions of stars in the Milky Way. When Bonaca analyzed the data she discovered an unexplained, jagged hole in GD-1, the largest star stream in the galaxy.
Star streams are like rivers of stars that move together across galaxies. Scientists believe they form when smaller groups of stars collide and a galaxy’s gravity stretches them into an almost single-file line. But scientists can find no explanation for a jagged-edged gap in GD-1, which looks as though something huge blasted through the star stream not long ago, its immense gravity dragging stars in its wake. Bonaca said she believes dark matter represents the most likely candidate for that mysterious something.
Whether dark matter, which astronomers cannot see or measure, actually exists is a matter of contentious debate. But many astronomers believe it offers the only plausible explanation for why many heavenly bodies don’t behave the way current knowledge predicts they should.
The massive size of the hole in GD-1, about a million times bigger than the sun, precludes the possibility of a rogue star causing it. And, if a supermassive black hole produced the gap, astronomers should see some evidence, like flares or radiation. —J.B.