Both work and are accepted. The sentence refers to some object and without context we don't know what gender that object is.
The "tips and hints" thing says to always use Det, if there is not antecedent. Is it wrong?
It says 'if it does not refer to a previous word'. In this case, den does refer to a previous word, that word is just not in this sentence, but it has been mentioned in the context or it's understood what it is.
The 'tips and hints' refers to expressions like Det är en bok where 'det' does not refer to the book, but is a placeholder subject to say 'the thing I'm now talking about'.
I used Duolingo on my laptop once purely so I could see these tips and hits and they weren't there?
In the desktop version - go to the duolingo website and there are many more resources
"Det" is used as a dummy pronoun or when you refer to an 'ett' word.
A dummy pronoun doesn't refer to anything, but is still required to make sense. For example, the 'it' in 'it is raining' is a dummy pronoun because it isn't referring to anything. It's there only because 'is raining' makes no sense on its own.
It is raining - Det regnar
I see the house. Do you see it? - Jag ser huset. Ser du det? ('det' is used because it refers to the house, which is an 'ett' word. If the noun were an 'en' word, you would use 'den')
So in certain cases "Den är" can be used for "It is" or only "Det är" is correct? I'm asking because I haven't seen "Den är" starting a sentence so far in the course.
Edit: Thank you to thorr18 below. There's no reply button there so :)
this refers specifically to objects within reach, in contrast to that.
den and it do not specifically reference something near or far.
The gender of what you are referring to differs and also if the object being referred to is unintroduced or unknown, you use det.
Not to worry! It's when the voice is saying "Jag äter dom" (phonetically; it's spelled "dem") you should be concerned. And even then it could just refer to several food items.
This is where knowing the gender of nouns is extremely important. "Den" refers to an "en" noun (en flicka, en pojke, and so on), whereas "det" is used for "ett" nouns or when the noun isn't known and hasn't yet been mentioned. It is also used for sentences like "it is raining".
Den - I don't see my newspaper. Do you see it? (Jag ser inte min tidning. Ser du den?
Det - My cat likes meat, but I don't like it. (Min katt tycker om kött, men jag tycker om inte det)