Just shine full-strength sunlight into every single cell of your body and you’ll never need to worry about viruses again.
(Baka to kurayami okkanai; “Both idiots and darkness are scary”)
The darkness is frightening because you don’t know who or what may be there or what may happen; you can’t see the dangers and therefore need to be especially on guard.
With an idiot, even if you have a good idea of their general modus operandi, motivations, and so on, there’s always a chance that they’ll latch on to some completely senseless idea without warning and cause unnecessary trouble. You can never quite know what they’ll do or say or what harm they may cause, and therefore need to be especially on guard. In other words, idiots are just as frightening as the darkness itself, and for similar reasons.
We begin with noun 馬鹿 (baka), “fool,” “idiot,” and the noun 暗闇 (kurayami), “darkness,” joined by the particle と (to, like “toe”), here acting as “and.” Any further particles are elided, but we may assume that this noun phrase is a topic, and that the comment on this topic is おっかない (okkanai), a Tokyo-dialect colloquialism for “scary.”
A related phrase replaces darkness with a bee/wasp nest (蜂の巣 = hachi no su, where hachi is a classification that combines both wasps and bees) and warns you not to touch it (手を出すな = te wo dasu na).
おっかない is an interesting term. Its etymology is not entirely clear, but it seems to have spread through eastern and northern Japan from the “Shitamachi” part of Edo (= Tokyo), and the leading theory is that it’s a slurred derivative of classical adjective おほけなし, “beyond one’s means,” “discourteous,” or even “awe-inspiring.” These days it seems not to be in common usage and has taken on a childish tone, perhaps by association with other verbal distortions found in Japanese baby talk.
(“Mattaku, Gen-kun tteba sakki arumihoiru de tsutsunda jagaimo wo chin suru tokoro datta, baka to kurayami okkanai na.”)
[“Sheesh, just now Gen was about to nuke a potato wrapped in aluminum foil. Idiots and darkness are full of dangerous unknowns.”]