star wars death star question?

the idea of the death star (im getting this from the EU expanded universe) was already thought up of during the naboo blockade, and the plans for this space station were complete by the battle of genoisis. in star wars episode 2, the attack of the clones the plans for the first death star were already completed, and at the end of episode 3 the revenge of the sith, you see darth sidious, darth vader, and governor grand moff tarkin hanging out on a star destroyer looking at its construction.

it was built even before the rebel alliance had ever existed. How did they know about the rebels before the rebels even existed??? they destroyed aldearan to spite and to interrogate princess leia organa, but they had no idea about her until much later on. Tarkin said that it was a means to replace the senate and to keep the local star systems in line, but i still dont get it. how did darth sidious/emperor palpatine/the galactic empire know about the rebel alliance in advance???