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One winter night in 1969, at an observatory atop Kitt Peak in Arizona, Michael Disney had a funny thought. As he peered into a huge, superluminous galaxy, he wondered: What if an alien astronomer there were staring right back?

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At the eyepiece of its own telescope, the intelligent extraterrestrial might likewise be ogling Disney’s smaller, fainter home galaxy, the Milky Way. Then another thought cut through the whimsy. The young Welsh astronomer realized the alien had no chance of seeing the Milky Way, let alone the universe’s oodles of dimmer galaxies.

Overwhelmed by the glare of all the stars stuffed into its resident galaxy, the alien would unknowingly be blinded to most of the cosmos. Disney wondered if we might be similarly deceived, awash in the inescapable glow of our own surroundings. “It occurred to me there could be a whole universe up there of hidden galaxies, just a little dimmer than those we can detect from Earth,” says Disney, an emeritus professor at Cardiff University in Wales. Since that revelation in the desert nearly a half-century ago, Disney, now 80, has searched for a shadowy galactic realm. His hunch gained momentum in the 1980s and 1990s, but at the turn of the century, the trail ran cold. Disheartened and defeated, Disney relinquished the hunt.

But recently, serendipitous sightings and new technology have reinvigorated the concept of a hidden cosmos. “Most of the universe is likely undiscovered,” says Greg Bothun, an astrophysicist at the University of Oregon who has long studied faint galaxies. The emerging population of dim galaxies likely outnumbers, and is strikingly different from, the typical bright galaxies we know and love, challenging our conventional theories of galaxy formation and evolution. Dim galaxies also may solve an old mystery about missing matter in the universe. By these reckonings, hidden galaxies are the cosmic norm, not our garish Milky Way and its ilk. Long overlooked, the dominion of dim galaxies may finally be getting its due.

Blinded By The Light Our universe is suffused with luminous galaxies. We can see the nearest few of these great collections of stars, gas (mostly hydrogen), and dust with our eyes from Earth. Telescope surveys suggest as many as 2 trillion are out there, albeit mostly of a small, faint, “dwarf” variety. Tidily, these galactic hordes come in stereotypical shapes and sizes, such as large spirals like the Milky Way, even bigger football-shaped ellipticals and those dime-a-dozen dwarf galaxies.

They follow typical life cycles, making abundant stars in their youths and slowing down as they age. For all we’ve learned about galaxies and the wider cosmos, though, astronomers have struggled with human limitations as heavenly observers. Our instruments can only readily perceive objects whose brightness contrasts enough with the glow of the night sky.

For sure, night looks dark — around 50 million times darker than day — but that’s still just relatively dark. “We live right next to this bloody luminous star called the Sun,” says Disney. “That’s always going to make it difficult for us to find this hidden universe.” The Sun’s brilliance affects astronomical viewing in two roughly equal ways. At night, an “airglow” lingers in our atmosphere as molecules radiate away the heat they soaked up during the day. To avoid airglow, we can send instruments into orbit, like the Hubble Space Telescope (for which Disney designed instruments).

But these spacecraft still must squint through the second of the Sun’s impacts, its bright illumination of icy and dusty particles around it, known as the zodiacal light. Add this to the copious light generated by all the other stars in our galaxy, and you get quite a glare. This natural “light pollution” extends to the entire electromagnetic spectrum, well beyond visible light. “We truly are imprisoned in our lighted cell,” says Disney.

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“It’s like you’re in the middle of a lighted room at night and you look out the window.” Your room’s light drowns out anything less bright. In 1976, seven years after his experience in Arizona, Disney wrote in a paper in Nature that our catalogs of galaxies are probably an unrepresentative subset of the true galactic population. A great number of dimmer and potentially sizable galaxies likely awaited discovery, he proposed.

Yet with little in the way of supporting data, the prophecy gained little traction. That changed a decade later, when astronomers stumbled upon a galaxy unlike anything they’d ever seen. A Giant Galactic Ghost Intrigued by faint blurs on old photographic plates of the Virgo galaxy cluster, a nearby region teeming with galaxies, Bothun and colleagues wondered if the apparitions might be smallish galaxies with “low surface brightness” — astronomer-speak for emitting less light per unit area than typical galaxies. Using Puerto Rico’s Arecibo radio telescope in 1986 to detect galactic hydrogen clouds, Bothun and colleagues uncovered a vast game changer of a galaxy a billion light-years away. Dubbed Malin 1, it’s been heavily studied ever since, and it remains the largest known spiral galaxy, seven times wider than the Milky Way with 50 times its mass. Yet, bizarrely, the galactic titan is rendered profoundly dim by its wispy spiral arms, spaced 10 times farther apart than in conventional spiral galaxies. “It’s impossible to understand how that object exists,” says Bothun.