In 2016, astronomers led by Pieter van Dokkum of Yale University printed a bombshell paper claiming the invention of a galaxy so dim, but so broad and heavy, that it have to be virtually solely invisible. They estimated that the galaxy, dubbed Dragonfly 44, is 99.99 % darkish matter.

A heated debate ensued about Dragonfly 44’s properties that continues to be unresolved. Meanwhile, greater than 1,000 equally massive however faint galaxies have turned up.

Dragonfly 44 and its ilk are referred to as ultra-diffuse galaxies (UDGs). While they are often as giant as the most important peculiar galaxies, UDGs are exceptionally dim—so dim that, in telescope surveys of the sky, “it’s a task to filter out the noise without accidentally filtering out these galaxies,” stated Paul Bennet, an astronomer on the Space Telescope Science Institute in Baltimore, Maryland. The vibrant star-forming fuel that’s ample in different galaxies appears to have vanished in UDGs, leaving solely a skeleton of aged stars.

Their existence has precipitated a stir in galactic evolutionary concept, which did not predict them. “They didn’t turn up in simulations,” van Dokkum stated. “You have to do something special to make a galaxy that big and faint.”

Wild new theories have emerged to clarify how Dragonfly 44 and different UDGs happened. And these large smudges of sunshine could also be offering recent proof of darkish matter’s invisible hand.

Too Much Dark Matter

As gravity brings clumps of fuel and stars collectively, their mixed energies and momentums trigger the mashup to inflate and rotate. Eventually a galaxy emerges.

There’s only one downside. As galaxies rotate, they need to come aside. They don’t seem to have sufficient mass—and thus gravity—to stay collectively. The idea of darkish matter was invented to offer the lacking gravity. In this image, a galaxy sits inside a bigger conglomeration of nonluminous particles. This darkish matter “halo” holds the spinning galaxy collectively.

One solution to estimate a galaxy’s rotation pace, and thus its darkish matter content material, is by counting its spherical clusters of stars. “We don’t know why, from a theory point of view,” Bennet stated, however the variety of these “globular clusters” correlates intently with these harder-to-measure properties. In the 2016 paper, van Dokkum counted 94 globular clusters inside Dragonfly 44—a quantity that implied a very giant darkish matter halo, regardless of how little seen matter the galaxy has.

No one had ever seen something prefer it. Van Dokkum and co-authors urged that Dragonfly 44 could possibly be a “failed Milky Way”: a galaxy with a Milky Way–sized darkish matter halo that underwent a mysterious occasion early on that robbed it of its star-forming fuel, leaving it with nothing however getting old stars and an enormous halo.

Or No Dark Matter

The object attracted the curiosity of one other camp of astronomers who argue that darkish matter doesn’t exist in any respect. These researchers clarify galaxies’ lacking gravity by tweaking Newton’s legislation of gravity as a substitute, an method known as modified Newtonian dynamics, or MOND.

According to MOND, the modified gravitational pressure for every galaxy is calculated from the mass-to-light ratio of its stars—their whole mass divided by their luminosity. MOND theorists don’t speculate as to why the pressure would rely on this ratio, however their advert hoc method matches the noticed speeds of most galaxies, with out the necessity to invoke darkish matter.

When information broke about Dragonfly 44, MOND advocate Stacy McGaugh, an astronomer at Case Western Reserve University, calculated from its mass-to-light ratio that it ought to rotate extra slowly than van Dokkum’s preliminary estimate indicated. The MOND calculation didn’t appear to suit the information.