Around 40 years ago, astronomers became aware that our galaxy, the Milky Way, was moving through space at a much faster rate than expected. At 2.2m kmh, the speed of the Milky Way through the cosmos is 2500x faster than a cruising airliner; 55x more than the escape velocity from Earth; and a factor of 2 greater than even the galaxy’s own escape velocity! But where this motion comes from is a mystery. Net motion can arise from nearby clumps in the distribution of matter, like a massive cluster of galaxies. The additional gravitational attraction of such a galaxy cluster can slow down, and even reverse, the expansion of the universe in its immediate vicinity. But no such cluster is obvious in the direction of the Milky Way’s motion. There is an excess of galaxies in the general vicinity, and an excess of radiation visible in X-ray telescopes. But nothing that in any way seems large enough to explain the results.

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