Dwarf Galaxies Challenge Dark Matter

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In the distant past, after the Big Bang, billions of galaxies formed. As early as the first half of the 20th century, astronomer Fritz Zwicky noticed that clusters of galaxies were moving too fast and suggested the existence of invisible mass; later, Vera Rubin convincingly proved that stars in many galaxies rotate as if held by something invisible. In honor of the discovery of the expanding universe, we should also remember Georges Lemaître, who laid the foundations of the Big Bang theory. According to the prevailing theory, each galaxy is enveloped by a huge cloud of dark matter—invisible substance that does not emit light but attracts ordinary matter with its gravity. This cloud acts like a thick elastic shell, protecting the galaxy from external influences. However, recent observations show that small satellite galaxies orbiting our Milky Way look strange: their shapes are distorted, and the stars inside them move faster than expected.

Elena Asensio and her colleagues decided to test this. They took precise data on the motion of eight classical dwarf satellites, obtained with the Hubble Space Telescope, and calculated at what distance from the center of the Milky Way gravity would tear these galaxies apart—this is called the tidal radius. Then they compared this radius with the actual sizes of the galaxies. In the standard model with dark matter, all satellites were safe: their protective halos were so large that the Milky Way could not harm them. But if we abandon dark matter and adopt the MOND theory (Modified Newtonian Dynamics), where gravity at large distances works slightly differently, the picture changes dramatically. For several galaxies, the ratio of size to tidal radius exceeded the critical threshold—meaning they should be noticeably stretched or even torn apart. That is exactly what astronomers observe: in satellites like Ursa Minor and Sculptor, stellar tails and distorted shapes have been found, and spectroscopy shows that their stars move more chaotically than expected.

These results make us wonder: is dark matter really necessary to explain what we see? If MOND theory is correct, then the familiar laws of gravity on large scales may be incomplete, just as Newton's laws are refined by relativity theory. Interestingly, tidal disruption may also feed supermassive black holes at the centers of galaxies by capturing stellar material. Moreover, alternative theories sometimes even reconsider the speed of light, but here the key change concerns gravity. The data remind us that astronomers have only recently learned to detect gravitational waves—ripples in spacetime that, like tides, carry energy. It is also important that these galaxies are virtually devoid of neutral hydrogen, so gas cannot explain their strange behavior.