What line is hidden behind the stars?
I will be uttering this with a moan
someplace aeons and aeons therefore:
$*****$ $*****$ $*****$ $*****$
and I picked up the one with inferior migration,
And this has caused complete disagreement
Thanks for this.
The Path Not Taken by Robert Frost.
I shall be telling this with a sigh
Somewhere, ages and ages hence:
Two roads diverged in a wood and I,
I took the one less traveled by
And that has made all the difference.
Much as I hate to add to or subtract from the verse above, @BigBlackBox and @LightnessRacesInOrbit have a point and an obscured line ought really to be part of an accepted answer. Any upvotes therefore properly belong to him. Technically, there should also be an explanation of the reasoning as well but there is pretty much a one-to-one mapping between the original and the obscured version. So:
A thoroughfare bifurcated in a copse and, myself-
The line is
A pair of highways uncoupled in a hard fibrous material, and I-
As Hugh Meyers' answer explains, the poem is an encoded version of the last stanza of The Road Not Taken by Robert Frost. The hidden third line asked for by OP therefore should be an encoded version of the third line
Two roads diverged in a wood, and I-
from that stanza, and not the original line as Hugh Meyers' solution claims.