[roywiggins]

If there was a lot of exotic elements out there, we'd have seen it with our telescopes. It would point to new physics, for a start, since we don't think very heavy elements are likely to form naturally at all, let alone be stable long enough to be observed in any quantity.

Even if such elements are out there in the universe in tiny quantities, we'd run into a billion carbon-based biospheres before we found any.

[saalweachter]

To elaborate on the first point, we can identify elements by the spectrum of light they produce. We discovered helium in the sun before we discovered it on Earth.

(This only works if the element is common enough and hot enough to emit light that is seen from Earth.)