When you think of a galaxy, you probably picture some gorgeous, sprawling spiral-armed disk loaded with bright blue stars and pink/red clouds of gas dotted along the arms. And in truth many galaxies are like that, including our Milky Way, while others are elliptical, or irregular, or even peculiar.

The common denominator is that theyre loaded with stars, millions or billions of them, so many that from a distance they blur together into a milky glow.

But recently astronomers found some galaxies that dont look like this at all. Located billions of light-years from Earth, they seem to defy what we know about galaxy structure. Almost no starlight is seen from them, and most of the light they emit is in very long wavelengths, far outside what the human eye can see. Theyre dusty that is, they have clouds made up ofgrains of iron, rocky, or sooty (carbonaceous) material but that dust is a lot colder than youd expect for a normal galaxy.

These weird galaxies have been a mystery for a while, but now a team of astronomers thinks they have the answer: These galaxies arent just dusty, theyre choked with dust, so much that they completely block the starlight coming from inside them. In fact, these galaxies are positively bursting with star formation, but its buried so deeply in opaque dust that these galaxies are dark in the kind of light we see. If they didnt have all that dust these galaxies would be blazingly luminous [link to paper].

The galaxies were found in deep survey observations of the sky. Theyre practically invisible even when observed in the near-infrared, just outside the visible spectrum, but at progressively longer wavelengths, from mid-infrared out to radio waves, they get brighter. If these were normal galaxies with a normal amount of stars making light and warming up the dust around them, theyd be brighter at shorter wavelengths of infrared. But theyre not.

Four such galaxies were known previously. The astronomers observed six more, all very far away; their light took roughly 12 billion years to reach Earth. Typically, to measure the galaxies properties, astronomers make some basic assumptions. For example, they assume the dust in star-forming clouds is thick enough to block visible light, but lets infrared light through. Thats usually a decent assumption.

But when they did that for these 10 galaxies they get contradictions and physical properties that dont make sense. Thats usually a good sign one or more assumptions youve made is wrong. So they then changed that assumption, and redid the math assuming the dust is very, very thick; so dense that not even infrared light can get out.

And suddenly the physics started making sense.

These galaxies are absolutely jam-packed with dust, so much so that even in infrared were only seeing the surface of these clouds. Its not so much these galaxies have more dust than usual, but that theyre small, so the density of dust is far higher. Normally infrared light can escape even from deep within a dust cloud, but in this case theyre so dense theyre opaque to it.

And that in turn means that to explain the amount of light we do see, these galaxies are cranking out stars, dozens of times the rate at which the Milky Way makes them. These are true starburst galaxies, even though, bizarrely, they emit no optical light we can see. Theyre dark galaxies.

OK, so thats just objectively cool; galaxies so thick with dust they veil what theyre doing inside. But this is actually important to understand. We measure the star formation rates of galaxies in various ways, but its a great way to understand what a galaxy is doing, how much gas and dust it has, and so on. The rate at which stars are born tells us a lot about the galaxy and also what the Universe itself was doing when the light we see left that galaxy, so sometimes deep in the past.

The fact that there are galaxies prodigiously churning out stars yet have been completely overlooked because theyre dark means weve missed a big piece of the early Universe; the astronomers estimate as many as 10% of all dusty galaxies in the early Universe are so dusty theyre dark.

The next question to answer is why theyre this way. Are these examples of galaxy collisions in the early Universe? Are the stars forming there under different conditions than in the nearby Universe, such that they make more dust? With only 10 sample galaxies known this isnt clear.

What is clear is that were still learning about what the distant, early cosmos was like, and that sometimes what we want to see is hidden from us until we find a clever way to see it. In this case a big chunk of star-forming galaxies were invisible. What else is out there weve overlooked?

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In the early Universe, dark galaxies swarmed - Syfy