For those that are asking about "Das sind" and who enjoy seeing relative links:
das ist/das sind
When it is used with the verb sein, the form das can refer to singular and plural nouns of all three genders:
Wer ist das? – Das ist seine neue Freundin.
Und das sind seine Kinder.
Das ist mein Wagen."
Similar for "Dies sind"
The das is not part of the noun phrase große Märkte -- in fact, it's not even the article "the", but a demonstrative pronoun "that" (well, "those" in the natural English sentence).
So there's no article at all in große Märkte and therefore strong inflection is necessary.
Using diese would be like using "These ones" -- you would only do so when you have a particular noun in mind even if you are not saying it. For example, if you are talking about the sights of a city and then you might say "these ones (= these sights) are big markets".
But if you're just introducing a new item with "this/that/these/those" to "point" to something, you'd use neuter singular in German -- das or dies, e.g. Dies sind große Märkte.