Let's step back more than 13.7 billion years. It's just minutes after the Big Bang and virtually all the matter that will ever exist is in the process of being formed. Two elements dominate them all: about 3 out of every 4 atoms formed are hydrogen and virtually all the rest are helium. The only other stable element created, lithium, was produced at a rate ten billion times lower. It seems like a tiny amount, but the quantity we see out there in our galaxy is even lower than that! Thanks to observations by the Very Large Telescope, we know that this discrepancy is not unique to the Milky Way.
There is a veritable mountain of evidence supporting the theory of the Big Bang, but few lines of support are more powerful than the observed abundance of elements in the universe. The 3-1 ratio of hydrogen to helium predicted by the model is more or less exactly observed in nature. It's this remarkable agreement that makes the so-called "lithium problem" so puzzling. Our model of the Big Bang is just as specific about the amount of lithium produced as it is about the hydrogen and helium, yet when we look out in the galaxy, we simply don't see as much as we think we should.
The best place to make these measurements is in very old stars, because it's likely that these objects formed from material that was relatively unchanged from its Big Bang origins. We've looked at a bunch of such stars in our Milky Way and they all seem to show this discrepancy, but we couldn't be sure whether this problem was specific to just us or not. Could the Milky Way have formed out of a giant cloud of material that was somehow light on lithium? We now know that this wasn't the case.
Recent observations of the globular cluster M54 are key to this. Unlike most visible globular clusters, M54 has an unusual characteristic: it's not in the Milky Way. Instead, it calls the nearby Sagittarius Dwarf Galaxy home. This small galaxy, made up of hundreds of thousands of stars, orbits the Milky Way but is clearly distinct from it. When astronomers looked at ancient stars in M54, they found similar levels of lithium depletion, confirming that the lithium problem is not confined to our galaxy.
So where did all that lithium go? The simple answer is that we just don't know. Scientists do have a couple of ideas, though. For one, we might just not be seeing all the lithium that's out there. It's not a particularly easy element to observe, so perhaps some is lurking out there unseen. Another possibility is that the lithium created in the Big Bang is being used up in the very stars we're observing. Lithium is destroyed at a relatively low temperature, easily obtainable in the centers of many stars. When this happens, primordial lithium from the Big Bang becomes other elements and we lose track of it forever.
Because of its incredible abundance, astronomers often think of the universe as simply made up of two elements: hydrogen and helium. From a bulk perspective, this is pretty much true, but, as the lithium problem shows us, nature is far more subtle and interesting than we often give it credit for.