This data is collected and updated nightly from over 5K websites from around the world.
The latest (not provided) % for Google traffic in the US is 85.95%.
To calculate your % of (not provided) traffic, simply take the count of (not provided) visitors in a period over the count of organic traffic from Google.
Danny Sullivan's history of "Dark Google" covers the year mark since Google went "dark" in Oct 2011. It's a great overview of why and how (not provided) came to be: Dark Google.
(not provided) shows in Google Analytics because no longer pass keyword referrer information from logged-in users. This means that publishers who use Google Analytics will no longer have access to the keywords that searchers used to reach their site, provided that searcher was logged into a Google account. Google originally announced this should impact 10% of all organic search queries.
SpyderTrap has a great timeline overview of the contributing events that have led to the rise of (not provided) in the past 2 years.
Below are some of the top articles surrounding this event.
How many keywords are affected?
Reactions from around the globe
Tips for dealing with (not provided)
Data visualizations created by SERPs.com founder Scott Krager