It would disappear from sight when it enters the darkness but function exactly the same otherwise
A creature with darkvision can't see through this darkness, and nonmagical light can't illuminate it.
Laser beams from a laser gun from the DMG are not a magical source. Thus the laser beam is also not magical. Since it is not magical it will be unable to illuminate the area covered by darkness.1
However, the spell only says that it is unable to illuminate the darkness, not that the laser ceases to exist. So, the laser beam would function otherwise like normal in the darkened area (eg it would hit and damage targets) and if it didn't hit something would come out the other side of the area the same as it went in (eg bright and lasery). The same thing would apply to, for example, a flaming arrow.
The power level of the light source doesn't matter at all - only the power source. You could place something with the illumination power of the sun itself in the center of the darkness and it wouldn't shine at all unless it was a magical light source.
1 - Obviously if this is using some sort of custom setting/item created by a DM which they do define laser shooting technology as magical, then the laser bolt would illuminate the darkness. However, by default the rules do not indicate that laser guns are considered to be magical at all.