This is a series of books about an interstellar human civilization. After losing a few colonies to unknown reasons, Humanity comes across these rogue asteroids traveling through space. They are made up of two parts traveling at slightly different speeds. In time when they catch up to each other, there is a huge explosion. By third or fourth book the protagonist figures out this is just some kind of fireworks display setup by aliens.

I remember it having a 70s/80s feel when I read it.

marked as duplicate by eshier, Edlothiad, Bellatrix, Valorum story-identification May 30 '18 at 20:22

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This is part of Jack McDevitt's Academy series; the first book is The Engines Of God

The plot you're talking about is a meta-plot through many of the books. There are waves of objects travelling through local space, dubbed Omega Clouds; they can somehow sense right angles.

Omega Clouds are guided by smaller objects in front of them, dubbed hedgehogs, which are basically made up of lots of right angle shapes. However, some of the Clouds have escaped their hedgehogs and are wandering through space, looking for right angles. They generally find them in civilised worlds, smashing into cities and devastating them.

In later books, the Academy theorise that they are part of an alien art show - the clouds make pretty colours when they hit, and seen from a spot way outside the galaxy, it looks like a work of art.

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