With all the splendor and beauty of the galaxies around us, it is easy to forget that such "normal" matter only makes up about fifteen percent of the matter in the universe.
The rest is a form of matter that does not interact with light at all, conveniently called "dark matter." Even though we cannot see it directly, it has helped to shape the universe into what we see today, and we can trace it using what we can see.
The most successful model of the universe's structure to date has dark matter forming "halos," or spherical constructs, coalescing at the junctions of a vast cosmic web, shown in blue above.
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In many of these halos, "normal" matter would clump together as well, forming the stars and galaxies with which we are so familiar. However, it has long been thought that some halos are too small to support star formation, and are thus barren of tell-tale stars and galaxies. How small is too small?
Astronomers using the ESA's Herschel Space Observatory came up with a clever way to answer that question. Herschel scans the sky from orbit in the far-infrared and sub-millimeter wavelengths, those longer than what our eyes can see. They looked at the most prolific star-forming galaxies at the most active time in the universe's history.