We often use "だ" in written language but not so often in spoken language.
There are two major formal styles of writing:
1. "です・ます調" ... Use "です" and "ます" as copula. This style makes polite and gentle impression. This style is used when we write to elders or venders to customers, for example.
2. "だ・である調" ... Use "だ" and "である". This makes serious and explanatory/argument-like impression thus used in such contexts e.g. dictionaries, newspapers and research papers. (The usage of "だ" is similar to "です" but there are some differences Nyagoslav said.)
Normally these styles are mutually exclusive.
Of course there are other rough ways to write Japanese
In spoken language, we rarely use bare "だ" because of it's too assertive impression, as some people said.
Instead of it, for example, we use "だね" which implies agreement.