finding a triple on the web

Dear all,

I am trying to figure out how it is possible to query n possible sparql
endpoints for a particular triple pattern.

I seem to think that a federated query provided by sparql only allows
explicitly mentioned sparql endpoints.

http://www.w3.org/TR/sparql11-federated-query/

It appears that powder allows filtering based on different parts of the URI:

http://www.w3.org/TR/powder-grouping/

Would I need some sort of web index that catalogues known triples that I
can then use a reference point for sparql querying?

Sorry about my lack of knowledge.

-Brent Shambaugh