Astronomers may have solved a long-standing mystery about how galaxies near the Milky Way move through space.
New computer simulations suggest that our galaxy is embedded in a vast, flat sheet of dark matter—a structure that helps explain why nearby galaxies behave the way they do.
The research, led by scientists at the University of Groningen and published in Nature Astronomy, focuses on the region just beyond the Local Group, which includes the Milky Way, the Andromeda galaxy, and dozens of smaller galaxies.
For decades, astronomers have puzzled over why most large nearby galaxies are moving away from us, even though the combined mass and gravity of the Milky Way and Andromeda should be pulling them inward.
The answer, the researchers say, lies in how matter—especially dark matter—is arranged around us.
According to their simulations, most of the mass surrounding the Local Group is not spread evenly in all directions.
Instead, it forms an enormous, pancake-like plane that stretches tens of millions of light-years across. Above and below this plane are vast, nearly empty regions known as cosmic voids.
This “large-scale sheet” of dark matter plays a crucial role in shaping galaxy motions. Galaxies that lie within the plane feel the gravitational pull of the Local Group, but this is balanced by the gravity of other matter spread throughout the sheet. As a result, these galaxies continue to move away from us in a smooth, orderly way that closely follows the general expansion of the universe. Meanwhile, regions where galaxies might be expected to fall toward us are mostly empty, making their influence invisible.
To reach these conclusions, the researchers created highly detailed computer simulations starting from conditions shortly after the Big Bang. They used information from the cosmic microwave background—the faint afterglow of the early universe—to set up realistic starting points. The simulations were then allowed to evolve over billions of years, eventually producing “virtual twins” of our local cosmic environment.
These virtual models successfully reproduced the current masses, positions, and speeds of the Milky Way and Andromeda, as well as the motions of 31 nearby galaxies just outside the Local Group. Crucially, the simulated galaxies moved in almost exactly the same way as the real ones we observe today.
The results offer the first detailed picture of how dark matter is distributed around our galaxy and how it influences nearby cosmic structures. They also show that galaxy motions alone can reveal the hidden arrangement of mass in our corner of the universe, even when that mass is invisible.
For astronomers, this discovery brings together two key pieces of the puzzle: the standard model of cosmology, which describes how the universe evolves overall, and the detailed dynamics of our local neighborhood. After many years of uncertainty, scientists now have a coherent explanation for why our cosmic surroundings look and move the way they do—guided by an enormous, unseen sheet of dark matter.
Source: KSR.
