"There are hundreds of lens galaxies that we know about, but almost all of them are relatively nearby, in cosmic terms," said Wong, first author on the team's science paper. "To find a lens as far away as this one is a very special discovery because we can learn about the dark-matter content of galaxies in the distant past. By comparing our analysis of this lens galaxy to the more nearby lenses, we can start to understand how that dark-matter content has evolved over time."
Dark matter, which can't be seen, makes up most of the universe's mass. Astronomers used to think that dark matter and stars increased at an equal pace as a galaxy grew, the researchers explained, but they now know that the ratio of one kind of matter to another changes.
The lens galaxy has presumably spent the past 9 billion years bulking up on dark matter as it grew up, and today it probably looks a lot like the massive galaxies we find closer to home. But because it's so far away, the image we see of it provides a fascinating lens into the past.