When you create a new ASP.NET Core 2 application and use the Authentication Mode “Individual User Accounts” (see below) a LocalDB database is created automatically.
These files are in c:\users\<you>\aspnet-Identity-<random>.mdf . They are intended for local development.
In Visual Studio 2017 you can access them via the SQL Server Object Explorer.
I like to use SSMS much better. Here’s how you can open these .MDF files in SSMS.
Step 1: Run SqlLocalDB.exe info from the Binn folder to get a list of instances
Step 2: Get the Named Pipe info by running SqlLocalDB.exe info MSSQLLocalDB
Step 3: Open the Named Pipe in SSMS. That’s in!