These days, scientists think they have a pretty good idea of what the universe is made of. By detecting the telltale signatures of various elements in celestial bodies like stars, asteroids, and other planets, scientists can estimate how abundant or rare each of the known elements is. Iron, however, has proven to be tricky to nail down.

We know iron is abundant on Earth as well as in stars, where the intense heat allows it to exist in the form of gas. That suggests that the element should also exist in relative abundance in the interstellar medium — the space between star systems in galaxies such as the Milky Way — but scientists haven’t detected much of it there. So, where is it? A new study published in the Astrophysical Journal might have the answer.

The “missing” iron isn’t really missing at all, the researchers say. The research team from Arizona State University in partnership with the W.M. Keck Foundation now says that it’s likely that the iron exists in the form of iron pseudocarbynes, which is a combination of iron and carbon molecules.

Carbon is known to be plentiful in interstellar space, and the team says that it would be easy to overlook iron pseudocarbynes when searching for it. These molecular chains of carbon and iron look identical to other carbon molecules from afar, making it impossible to tell how much of the carbon that scientists can see is actually hiding iron as well.

“We calculated what the spectra of these molecules would look like, and we found that they have spectroscopic signatures nearly identical to carbon-chain molecules without any iron,” Pilarasetty Tarakeshwar of ASU’s School of Molecular Sciences said in a statement. “Previous astrophysical observations could have overlooked these carbon-plus-iron molecules.”

If the theory proves to be correct, the iron that scientists have been looking for between star systems is essentially hiding in plain sight, we just can’t see it.