I was reading 'The Clone Wars-Wild Space' the other day and the book depicts how hard it was for Obi Wan to even go near a Sith temple which contained ancient Sith artifacts including a holocron, and Obi Wan was nearly driven mad.
But Sith lords like Palpatine didn't even feel discomfort at being so near the Jedi Temple, which was a concentration of light side power.
Is this explained in the EU?
The answer depends on which version of the Force you subscribe to - even in-universe, there are two largely different interpretations of the nature of the Force.
The first, and the in-universe historical interpretation of the nature of the Force is that there is no "light side". Before the Jedi and the Sith, there were only Je'daii. The Je'daii's ancient name for the light side - Ashla - was originally used as the name of the Force in its entirety. This led some to believe that there is only the neutral Force - mistakenly called the "light" - and the dark side, an unnatural corruption of the neutral, natural Force. As such, dark side practitioners will not feel discomforted by the presence of the natural Force - the neutral Force is a fundamental part of all that exist.
Edit: An analogy is to imagine your own body as the neutral Force. Sometimes, aged cells will mutate and become cancerous (falling to the dark side - die a hero or live long enough to become the villain). It continues to exist within your body (the neutral Force) unharmed while progressively corrupting the cells around it and turning them cancerous too, until the body naturally reacts and creates macrophages (the Jedi) to destroy these cancer cells (bringing the Force back into balance).
The second, more recent (in-universe-wise) interpretation, as demonstrated by those such as the Father and his Children, is that the Light Side is distinct from the Dark Side, the two required to be in balance as counterparts to collectively make up the neutral Force. In this aspect, the light side has clear properties defined:
Edit: The implications of the second interpretation to a dark sider needs correction.
Light side energy is indeed capable of countering the dark side, but it requires an active effort. Two examples are described in Luke Skywalker's biography:
Without active effort from its practitioners, however, naturally occurring light side energy will not passively "purify" any dark sider present, while the dark side does passively tempt and corrupt any light sider present. That's where the light side comes into play - to guide and help its followers to resist passive and active attempts by the dark side to corrupt.
So we establish that a dark sider is unaffected while in a place strong in the light unless it is being actively directed at him. This can only happen if said dark siders, like Palpatine, can pass through unnoticed by light siders around him - your enemy will not actively target you if you don't pull aggro, right? This can be achieved through Force Clouding:
There where two methods of Force clouding. The first and most common method was channeled through the mind. This was done by the individual being him or herself and allowing those around him to take them for granted. The practitioner would submerge themselves in the unified pattern the Jedi were attuned to; visible in the Force but not as a Sith. In this method of hiding, a Sith would be able cloak himself but still be able to attack without warning if needed.
The second method was accomplished through the use of Sith Sorcery and involved the user casting a complicated spell on themselves. Force clouding by this method required achieving a balance between strength and delicacy. This method was used when a Sith wished to project an aura of light-side energy so they could pass themselves off as being a Jedi.
In other words, there are two levels of "potency" to Force Clouding:
Off-topic sidenote: These two differing interpretations in-universe is one of the key roots of out-of-universe debate over what Anakin's role of "bringing the Force back into balance" really means - is he meant to destroy the power of the unnatural dark side completely (what George Lucas intended), or is he truly meant to bring both the light and the dark back to square one (what EU is implying in the long term)?
Using only legends sources -
In the New Jedi Order book Traitor by Matt Stover it is revealed that the Jedi temple was built atop a "wellspring of dark energy"
This would have corrupted the power of the light side and clouded the vision of the Jedi who lived there, this would most likely make it less painful for Sidious to be there.
It is also mentioned in the book Darth Plagueis by James Lucerno how Palpatine is able to shield himself so he doesn't show any force sensitivity.
One might conclude that this talent would work the other way and shield him from any effects the light side might have on him. It would also explain why the Jedi council could not sense him.
