Ever seen the shadow of a galaxy? Just hold that galaxy between two fingers and place it in front of a light. It may seem impossible, but these shadows are actually one of the most powerful ways to learn about galaxies. Because, when you look at a galaxy through a telescope, what you see is starlight.

But the starlight you see doesn't tell you the whole story about that galaxy: it only shows you the stars. And stars only make up a tiny fraction of the content of these galaxies. In fact, this is one of the biggest problems in astrophysics today: because of it, we don't really understand how galaxies work. What we need a way to reveal all the hidden gas around galaxies

Wouldn't it be great if we could place a backlight behind a galaxies, to reveal the invisible gas through its shadow? Just like a watermark on a dollar bill? Well, It turns out there already is a backlight in the Universe, shining through all the galaxies. That light turned on a very long time ago. It turned on with a bang. A very big bang... That light is the afterglow, the leftover heat from the big bang.

It's everywhere, but it is really faint. So to observe it, my collaborators and I used one of the most powerful telescopes in the world, in the Chilean desert. We stared at a galaxy and searched for its shadow. But we couldn't find it: it was too faint to detect!

That's when I remembered the little song you just heard a minute ago, remember? That galaxy was only one of millions and billions, in this amazing and expanding Universe! So I used the massive supercomputer here at the lab to accumulate the shadows from many many galaxies, more than 100k of them, and slowly as I added more and more galaxies the shadow came into focus! The result is what you see here! This dark blob is the most precise shadow image of its kind!

An image like this one tells us a lot about how galaxies work. This shows that around every tiny galaxy in the Universe, there is a gigantic gas cloud. This gas cloud is the key to understanding galaxy formation. Now this is only the beginning: New telescopes are setting out to observe shadows of tens of millions of galaxies, with unprecedented precision. And this will transform our understanding of galaxy formation. So next time you look at a beautiful picture of a galaxy, remember: you really haven't seen a galaxy until you've looked at its shadow.