As the sun (and Earth) drift through the local interstellar medium, it encounters varying concentrations of dust, gas, and debris... Scientists have been able to map out (in a general manner ) the relative densities of the gas and dust out past several hundred light years in all directions. "Bubbles" of lesser density gas/dust have been discovered and it's known that these bubbles are formed from an outward pressure - in many cases, the pressure from local supernovae. Similar bubbles have been found in other regions filled with large, hot stars emitting copious amounts of energy.

Reference the diagram below (from Sky and Telescope March 2007). Even though our sun is in a lesser density area, it still passes through large amounts of galactic dust clouds. For comparison, you'll note that the Pleiades bubble is also a lower concentration of interstellar material. Yet, it's filled with dust streamers. To see the picture, click HERE.

As our solar system passes through the streamers of dust and gas that exist (everywhere) throughout our galaxy, the changes in the density will have significant influences on the amount of radiation (from outside the solar system) and the amount of sunlight that falls on the Earth's surface. This in turn impacts our environment and climate in ways we haven't ever accounted for or even knew existed until recently. Without doubt, none of this has been accounted for in any climate modeling yet the impacts are significant.