Until 2019, the kilogram was defined by a physical platinum-iridium bar kept in a vault in France.

The Problem

The problem was that "Le Grand K" was losing mass over time (about 50 micrograms in 100 years). In high-precision science, this was unacceptable.

The Solution

Now, the kilo is defined via the Planck constant (h), an invariant value of the universe. Now, the kilo is the same everywhere, even on the other side of the galaxy.