United States court cases are commonly cited like this:
Obergefell v. Hodges, 576 U.S. ___ (2015)
So... what is "___" here?
Underscores are often used to indicate that the particular case has not yet been assigned to a specific volume (preceding the reporter abbreviation) or page (following the reporter abbreviation).
In your example, the case will appear in volume 576, but the exact page has not yet been determined.
This is a citation of a slip opinion, an opinion that has not yet been bound into a volume. A permanent reference is not yet possible, so the blank lines serve as placeholders for the information that will be added when it becomes available.
For this reason, the page number of the slip opinion is provided, when it is required, in parentheses after the placeholder citation. For example:
See United States v. Windsor, 570 U. S. _, _ (2013) (ALITO, J., dissenting) (slip op., at 7).