Space

Full disclosure: He does acknowledge that the most likely explanation is that it's not aliens.

"Misinterpretation is obviously the most boring on that list, which perhaps means it's the most likely to be the case. ... No one's ever actually put this stuff in a 10,000-Kelvin oven and measured how they absorb light. So the characteristic frequencies are theoretical, and it's quite possible that they are slightly wrong, and instead of seeing radioactive elements, we are instead seeing a far more mundane set of elements just behaving in an unexpected way."