This question already has an answer here:
It was previously known to the Republic (and, therefore, it stands to reason, the Empire) that Dagobah was one of several major unexplored Force-sensitive planets.
Once Yoda and Obi-Wan went into hiding, why didn't Palpatine send thousands of probe droids to scour those planets? He was meticulous about cataloging dead Jedi from Order 66, plus he knew personally that Yoda and Obi-Wan had survived based on personal interaction as well as the story from Vader.
Wouldn't he have realized the threat that Yoda posed as a teacher to any other survivors and wanted to find him at all costs?
Was it simply a matter of Yoda's being able to mask his Force signature with the dark side cave?