It emerged in April — a dark void, encircled by a luminous ring, humanity’s first-ever look at a black hole. The existence of black holes had not been in doubt, of course: Astronomers have long studied stars whipped around by invisible slings, and in recent years they’ve even listened to the ripples of black holes’ spacetime-shaking collisions. But there’s something different about seeing the emptiness, the tear where the universe should be — the absurd, unforeseen consequence of Einstein’s century-old theory of gravity, made visible.

That theory of gravity also had a big year. Who would have thought that in 2019, exactly 100 years after general relativity was spectacularly confirmed, we’d still be arguing about its consequences? But right now, perhaps the biggest mystery in the universe is the discrepancy — small, but increasingly nettlesome — between two complementary ways of measuring the universe’s expansion. The hope is that by homing in on exactly what is causing the disparity, physicists will be able to figure out exactly where our ideas about the structure of the universe go wrong.

Then there are the mysteries where hard evidence is harder to come by: Where did the universe come from? How could time emerge out of nothing? Is the entire cosmos just a hologram — a projection from another dimension? It’s possible that we’ll never know the answers, but it is remarkable how much physicists can figure out, just by following the fertile footpaths laid out by Einstein and the founders of quantum theory.