Great question — answering as the UphillTech team:
To see the dark side difficult it is. Palpatine is using several techniques such as force stealth. From wookieepedia Force stealth also referred to as Force Concealment or Buried Presence was a power that was used by highly skilled Force-sensitives to mask their Force alignment (Light or Dark), their ability to use the Force, or even their entire presence from other Force-sensitives.
Full answer by OldPadawan: https://movies.stackexchange.com/a/120048 (CC BY-SA 4.0).
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