Is it illegal to carry or conceal a handgun in a movie theater in Indiana?

No. There are a few places listed as off limits, but movie theaters is not one of the places, but generally speaking, anyone that owns or controls a property can make rules regarding entry, so a manager or owner may prohibit guns on the property. Whether or not it is an actual violation to carry anyway, or what they have to do legally to inform patrons of the rule depends on how state law is Worded. I didn't any specific requirement by Indiana law regarding that. See related link for more information or check with a local authority.