How might my futuristic interstellar civilisation have missed a Dyson swarm on their doorstep?

It's the future. The setting is about as "hard-ish sci-fi" as it is possible to be, with the one big exception of a viable FTL engine. Humans have began spreading out into the galaxy, colonising nearby star-systems that have compatible exo-planets. There are currently about 100 colonies, spread over a roughly 1000ly wide region. Travel times between stars frequently take multiple months. Humans still appear to be alone in the galaxy, but some questionable ruins of more primitive civilisations have been found.

The plucky humans are about to have a surprise run-in with a vastly more powerful civilisation, who instead of setting out into the galaxy, decided to stay at home and construct a Dyson swarm. The star hosting this swarm is only a hundred light years or so outside human controlled space. They built it a long time ago, so nobody on earth would have noticed it under construction. Never-the-less, I would have thought that the spectral lines and luminosity of a star with a few billion giant discs orbiting it would be different enough to be visible by future astronomers.

How and why did my future astronomers manage to miss this feat of stellar engineering on their (relative) doorstep?

I would expect an answer to fall into one of the following categories:

1. Human astronomers saw the unusual characteristics of the star, but dismissed it as a natural phenomenon for some reason.
2. The Dyson swarm builders constructed their swarm in such a way as to hide the fact that it is an artificial structure when seen from interstellar distances.

posted almost 5 years ago

MadScientist‭

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