Translation:Astronomy contains a lot of mathematics.
You are right about the article. Ending in 'a' means definite article. The problem is that English allows zero article to refer to plurals and mass nouns and abstract terms without making them indefinite. In any other language with indefinite articles the absence of an article would automatically make it indefinite:
Men go to work
Barbatii merg la munca (The men go to work)
Because Barbati merg la munca would not be allowed.
In fact is unlikely to find a zero article noun as a subject in a romanian sentece.
Looking forward to he proven wrong.
Depends on the context. The sentence ”Bărbați merg pe stradă.” it is actually a perfect correct sentence and it means ”There are men walking on the street.” The more natural way to say it is ”Pe stradă merg bărbați.”, but it has exactly the same meaning. The word order in Romanian is quite liberal.
This course (like many others on DL) really needs ”syntax lessons”.
"Astronomy contains much mathematics" still marked wrong. So is the Romanian "matematica (+accent) genative? No... Then much mathematics is surely not just ok but better than of mathematics?
Note neither is natural English. Astronomy uses / makes use of / relies on would be natural, contains - sounds foreign and sounds wrong as an English phrase.
Duolingo, I'm losing confidence.