A mysterious blast of radio waves has been traced to its source. Its origins suggest there are multiple causes of these signals, which have been puzzling astronomers for nearly 10 years. What’s more, it has given us the first direct measure of the density of the interstellar medium, confirming there is a whole lot of matter we can’t account for.

Fast radio bursts (FRB) are powerful but incredibly brief, so are hard to spot. The Parkes radio telescope in Australia made the first detection in 2001, but no one noticed the signal, lasting just 4.6 milliseconds, until 2007. Since then, astronomers have only found a dozen or so more, and just one – in May 2014 – was seen as it happened.

Even then, we couldn’t identify the source. “When you find them you have an idea of where they are on the sky, but there are lots of galaxies so you’re not really sure which one it is from,” says Evan Keane at the Jodrell Bank Observatory in the UK.

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