Our cosmos seems, at times, like one impossibly vast factory, just pumping out mysteries.

The deeper we peer, the more we learn – but each new discovery stirs up a plethora of new puzzles to solve, like a swarm of fascinating science bees. Meanwhile, some burning questions have persisted across decades, resisting our best attempts to solve them.

Here's a selection of some of our favorite mysteries the Universe has to offer.

The Hubble Tension

We know, from several separate lines of evidence, that the Universe is expanding. What measurements can't agree on is the speed at which it is doing so.

There are two main ways to measure the speed of the Universe's expansion, known as the Hubble constant .

The standard ruler approach uses relics of the early Universe. These are things like t

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