dmesg log is the place where system information are written to it, similar to /var/log/messages. It is particularly useful to get information of Hardware details. But after some time, the information in /var/log/dmesg.today & /var/log/dmesg.yesterday are replaced with system messages where by boot messages are replaced.
Fortunately, a copy of boot messages in dmesg are still available in :
/var/run/dmesg.boot
Au revoir !!!