Molecular Clouds

Even the "densest" parts of molecular clouds are still around 10²–10⁶ molecules per cubic centimeter. Earth's atmosphere at sea level has about 10¹⁹ per cubic centimeter. So a dense cloud core is still a far better vacuum than anything achievable in a lab on Earth. If you were floating inside one, there'd be no sensation of "being in a fog" — no pressure on your skin, no haze swirling around you at arm's length. Locally, it would look like ordinary, essentially empty space.

What you would notice is the darkness. The reason molecular clouds like Barnard 68 look like solid black holes in the starfield from outside isn't gas density - it's dust. Tiny solid grains, mixed in at something like one grain per thousand cubic meters, scatter and absorb light so effectively over light-years distance that they black out everything behind them. From inside, that effect is even more pronounced: you'd be surrounded by dust extinguishing starlight in every direction, so the sky would be close to fully black — none of the ambient starlight you get even on a moonless night on Earth, because the cloud is swallowing the light from everything beyond it.

Any stars that are especially nearby would appear at first as a feint reddish glow, growing gradually brighter as you approach.

Even at very high speeds (in striker terms, moving at speed(vacuum)=15 or so), the molecular density is not enough to to form a shockwave like during Earth re-entry by a spacecraft. However at these speeds another issue shows up. Each individual hydrogen atom or dust grain you hit carries enough kinetic energy, relative to your ship, to act like a tiny bullet or a cosmic-ray particle. Ultimately, you get cumulative sandblasting and radiation damage: surface erosion, pitting, secondary particle showers, and localized heating at the impact sites, potentially eroding or perforating a hull over time even though the surrounding medium is a near-perfect vacuum by everyday standards.