A new study suggests that a mass of dark matter has distorted our galaxy, the Milky Way. Initially, scientists believed our galaxy to be a flat disk dominated by two spiral arms that drag stars around a central point. Measurements taken since the mid-20th century reveal that it is, in fact, inexplicably warped.
The deformation primarily occurs at the edges of our galaxy. Some regions bend downward while others expand upward. Computer simulations may have uncovered the cause. A mysterious event has misaligned the invisible part of dark matter in our galaxy. Scientists published their findings on September 14th in the journal Nature Astronomy (ref.).
“These results, in combination with data regarding the stellar halo, provide compelling evidence that our galaxy is embedded in an inclined region of dark matter” researchers wrote in the study. Dark matter is a mysterious and somewhat paradoxical type of matter. It makes up 85% of the universe’s matter, but because it does not directly interact with light, it is completely invisible.
However, scientists can observe its gravitational effects on the surrounding environment. Dark matter reveals its presence by accelerating stars to otherwise inexplicable speeds as they orbit galactic centers. It distorts distant starlight and shapes the galactic halo of the Milky Way.
The galactic halo is a vast sphere of stars that floats like leaves on a pond of dark matter beyond the spiral arms of the Milky Way. In a 2022 study, astronomers examined this region using the European Space Agency‘s (ESA) Gaia spacecraft. The probe maps the positions and movements of approximately 2 billion Milky Way stars. By closely examining Gaia’s data, they discovered that the stars suspended in the galactic halo were oddly out of place (ref.).
What does it mean for a stellar halo to be skewed due to the dark matter it is suspended in? Researchers used a computer model to recreate a young galaxy similar to the Milky Way with a dark matter halo tilted 25 degrees relative to its disk. After simulating for over 5 billion years, researchers found they had created a galaxy very much like our own.
“We show that inclined dark matter in the same direction as the stellar halo can induce a deformation and a flare in the galactic disk with the same amplitude and orientation as the data” researchers wrote. What caused the dark matter to fall out of alignment around our galaxy is unclear. Researchers’ simulations suggest it was likely a massive collision.
Likely, part of another galaxy crashed into ours. This collision may have caused a tilt in the dark matter halo of up to 50 degrees before slowly settling into the current elevation angle of 20 degrees.