The universe is flat — to within 0.4% precision — and nobody fully understands why
In cosmology, the "shape" of the universe refers to its spatial curvature. It could be positively curved (like a sphere), negatively curved (like a saddle), or flat (like a sheet of paper). Measurements of the cosmic microwave background radiation — the afterglow of the Big Bang — show the universe is geometrically flat to within 0.4% accuracy. This requires the total energy density of the universe to be almost exactly at the critical value. The precision is so extreme that physicists call it the flatness problem: it looks designed.
The universe being flat to this precision is like throwing a dart from 300km away and hitting a bullseye smaller than an atom.
“The universe is geometrically flat to within 0.4% precision — and no one knows why it's so perfectly tuned.”