rosbag doesn't display map in real-time on rviz [closed]

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I am following the gmapping tutorial, and it says:

If you don't care to wait until the log playback and mapping process has finished before seeing some results, then you can watch the progress in rviz.

rosrun rviz rviz
Add a display with a map, set to the topic /map

I have rosbag of sample data running in a different terminal. I added a display to rviz with topic /map, but it doesn't seem to show anything. There is an exclamation mark beside "Message".

Also, when I type in rqt_graph, no messages are being published to topic /map.

How can I resolve this? I appreciate any help as I am new to this.

Thanks