All of Alaska does not go dark for 6 months. But the further north you go, the darker it gets.
Sources suggest that Barrow (located on the northern tip of the west coast of Alaska) may experience 67 days of darkness in winter (November 18 - January 23). People in Barrow may enjoy the midnight sun all summer - over 80 days of uninterrupted daylight.
Anchorage enjoys a more modest amount of daylight in summer than Barrow - but that's still a good 19 hours between sunrise and sunset on the longest day of the year, Summer Solstice.
Planning to go to Alaska? Find out when the sun will rise and set during your Alaska trip using this Alaska Daylight Hours Calculator from Alaska.org.