Spherical domain walls and vacuum bubbles can spontaneously nucleate and expand during the inflationary epoch in the early universe.
After inflation ends, the walls and/or bubbles form black holes with a wide spectrum of masses. For some parameter values, the black holes can serve as dark matter or as seeds for supermassive black holes at galactic centers. This mechanism of black hole formation is very generic and has important implications for the global structure of the universe.
