Google has off-loaded static content (Javascript/Images/CSS) to a different domain name in an effort to reduce bandwidth usage and increase network performance for the end user.
https://google.com
Content Delivery Network
Get access to data on 39,150,403 websites that are GStatic Google Static Content Customers. We know of 2,000,001 live websites using GStatic Google Static Content and an additional 37,150,402 sites that used GStatic Google Static Content historically and 1,267,144 websites in the United States.
Get a list of 39,150,403 websites using GStatic Google Static Content which includes location information, hosting data, contact details, 2,000,001 currently live websites and an additional 4,896,047 domains that redirect to sites in this list. 37,150,402 sites that used this technology previously and 1,267,144 websites in the United States currently using GStatic Google Static Content.