Shen thought of a way to check: If there are two white dwarfs rotating around each other, and one explodes as a supernova, nothing will be left to hold onto the other. Like a swinging lasso that’s suddenly released, it should fly away as a “hypervelocity” white dwarf.

If the D6 theory is correct, hypervelocity white dwarfs should be common. If it’s wrong, there should be none.

The opportunity to test the scenario arrived in 2018, when the European Space Agency’s Gaia space telescope released a massive new census of objects in the Milky Way. On the day of the release, Shen and his team stayed up all night analyzing the data. They found three fast-moving white dwarfs. Not many, and not none. This was troubling.

Simulating Supernovas

Around this time, multiple teams set to work on computer simulations to test the D6 hypothesis.

Shen and colleagu