We don't want to panic you or anything, but it's possible that another galaxy is currently crashing into the Milky Way... and that it's not the first time that it's happened.
The Unidentified Crashing Object colliding with our galaxy is called Smith's Cloud, and is a cloud of hydrogen discovered last year. At the time, scientists believed it had a mass of a million suns, but it's been theorized that it would have to be much larger to have enough gravity to survive the impact. University of Sydney, Australia's Matthew Nichols and Joss Bland-Hawthorn have calculated that Smith's Cloud may, in fact, be up to 100 times larger than originally estimated.
Before you get too worried about the end of all known existence - or, at least, a Crisis On Infinite Earths - take some comfort in the knowledge that, if theories are correct, this has all happened before; according to the trajectory of Smith's Cloud, it would have impacted the Milky Way 70 million years ago. And it's not like anything has changed since then, right? Well, nothing important.
Dark galaxy crashing into the Milky Way [New Scientist]