Shields used to create aerodynamic bubble for ships in atmosphere.

[Kamen Rider Blade]

I honestly don't mind that they don't have visible exhaust

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When not using Afterburners, most Aircraft don't show exhaust either unless you're in very specific conditions.

In the vaccum of space, there's probably nothing to see realistically given the nature of a vacuum.

And StarFleet probably does a few tricks to hide it's exhausts.