Decay

[robsharp14]

Is there an element that does not decay but can be identified from a extremely far distance or an element with a multi-billion year half life

[robsharp14]

uranium-238

[robsharp14]

my theory is way too risky

[Borek]

Spectroscopic methods are not distance related - assuming source is bright enough we can research its composition from any distance. That's how we know there are the same elements in other parts of the Universe.

[SpectraCat]

Not really sure what you are after here ... most of the elements in the periodic table have at least one stable isotope that does not decay.