There is no important difference. You could use either, based only on what sounds good.
That said, "back" does subtly imply a change in context. When you say "back in 2003," you are implying that something about 2003 is different from the present day. Now, obviously each day is different from the one before, so it's never wrong to use "back in___", and obviously each day is similar to the next, so it's never mandatory either. But if you're going to describe some key difference, it often sounds better. For example, "EVE launched back in 2003, when the idea of a space MMO was fresh and new."
There are a few other uses which work similarly, like the cliche "Meanwhile, back at the ranch." (which indicates we have switched from some cowboy's exploits to a separate scene with different characters).
A very similar usage of "back" instead refers to the audience's focus returning to something from earlier in the presentation. You hear this used often on reality shows with multiple teams: "Back at the Johnsons' house, Zed has run into some problems."
Note also that "in" doesn't necessarily indicate past tense. Example: "In March of 2020, we will begin phase three of our master plan."