repository - Which Ubuntu repositories are totally safe and free from malware?

I read in the news about all the malware that is infecting Android OS. The malware is in Google's App Store and people are unknowingly downloading and installing it.

As I understand, Ubuntu's Main repository is safe for me to download from (I won't be infected with malware from doing so) because Canonical engineers review the software. But what about other repos, most notably the Universe repository? Does the Universe repo receive any sort of review to protect from malware? Is it advisable to avoid the Universe repo for fear of unknowingly downloading malware from it?

I've read PPAs are particularly dangerous because they are not reviewed. I'm assuming it is perfectly safe to use the Google Chrome PPA however.

So if I use nothing but the Main & Universe repositories and Google Chrome PPA, will I be protected from unknowingly downloading malware?

If Ubuntu does gain hundreds of millions of users like Mark Shuttleworth predicts, won't Ubuntu PPAs become the malware problem for Ubuntu like Google's App Store is today for Android?