A Margulis spacetime is the quotient of the Minkowski space by a free proper orientation-preserving isometric action of a free group of rank at least two.

From Danciger, Kassel, and Guéritaud:

"Based on a question of Margulis, Drumm–Goldman conjectured in the early 1990s that all Margulis spacetimes should be tame, meaning homeomorphic to the interior of a compact manifold."

In a series of paper, I believe Choi, Drumm, and Goldman, and independently Danciger, Kassel, and Guéritaud resolved this conjecture affirmatively.

Links:

1. Topological tameness of Margulis spacetimes, by Suhyoung Choi, William Goldman
2. Tameness of Margulis space-times with parabolics, by Suhyoung Choi, Todd Drumm, William Goldman
3. Geometry and topology of complete Lorentz spacetimes of constant curvature, by Jeffrey Danciger, François Guéritaud, Fanny Kassel
4. Margulis spacetimes via the arc complex, by Jeffrey Danciger, François Guéritaud, Fanny Kassel