A mᴀssive blob of invisible dark matter warped our galaxy, a new study says.

Since the mid-20th century, measurements have shown that the Milky Way is warped bizarrely out of shape. Scientists previously thought it was a flat disc with two spiral arms trailing stars from a centre bar.

Our galaxy warps largely near its boundaries, where some portions bend downward and others flare upward, resembling a smashed sombrero. Computer models may have uncovered the cause: a mystery event that misaligned our galaxy’s dark matter halo. Researchers reported their findings in Nature Astronomy on Sept. 14.

“These results, in combination with stellar halo data, provide compelling evidence that our Galaxy is embedded in a tilted dark matter halo,” the report stated.

Dark matter is strange and paradoxical. Although it makes up 85% of the universe’s matter, it is invisible because it doesn’t interact with light.

Scientists can observe its gravitational impact on surroundings. Dark matter accelerates stars to unfathomable speeds as they orbit galactic centres, warps distant starlight, and shapes the Milky Way’s cosmic halo.

The galactic halo, a large sphere of stars floating like leaves on a dark matter pond, lies beyond the Milky Way’s spiral arms. The European Space Agency’s Gaia probe, which tracks the Milky Way’s 2 billion stars, was used to analyse this region in 2022. They found oddly off-kilter stars in the cosmic halo by analysing Gaia’s data.

The researchers created a computer model of a young Milky Way-like galaxy with a dark matter halo tilted 25 degrees from its disc to test the effects of an unbalanced stellar halo on its dark matter halo. Researchers uncovered a galaxy comparable to ours after modelling it for 5 billion years.

“Here we show that a dark halo tilted in the same direction as the stellar halo can induce a warp and flare in the Galactic disc at the same amplitude and orientation as the data,” the researchers stated.

Researchers’ models imply a mᴀssive collision, perhaps from another galaxy hurtling towards ours, caused our galaxy’s dark matter to tilt. The dark matter halo may have tilted about 50 degrees before slowly falling to its current 20-degree angle after this impact.

Reference:

Han, J.J., Conroy, C. & Hernquist, L. A tilted dark halo origin of the Galactic disk warp and flare. Nat Astron (2023). DOI: 10.1038/s41550-023-02076-9