Considering the economy and face-value price of the tickets, that shouldn't come as a total shock. The Super Bowl is often an automatic sellout with big corporations buying up bug chunks, but with a cold climate city and the country's financial struggles the ticket market is still active.
If you live close and want to make a last-second purchase, the options are out there. A great place to go is tiqiq.com where seats are still being sold.
It's a little surprising with the New York and New England markets that fans haven't bought up all the excess tickets with Indianapolis being a reasonable drive away.
The capacity of Lucas Oil Stadium is 67,000, so it's not an overly spacious stadium by previous Super Bowl standards. Regardless, it should be a loud atmosphere with the roof being closed tonight.