In medicine/biology: the eclipse period in a virus life cycle when viral particles are not yet detectable.
ウイルスは感染後しばらく暗黒期を経て増殖を始める。
After infection, the virus goes through an eclipse period before starting to replicate.
A dark age: a period of cultural, economic, or technological decline or stagnation; used literally or metaphorically.
その国は長い暗黒期を経て、やがて復興した。
The country went through a long dark period and eventually recovered.
経済の暗黒期が続き、失業率が上がった。
A period of economic stagnation continued and unemployment rose.
近代以前の『暗黒期』という表現は議論の余地がある。
The term 'dark age' for premodern times is debatable.