Until recently, astronomers believed that the particles in a meteor stream became more dispersed at the edges but remained dense at its center. A new experiment, however, used computer-tracking software to extrapolate the density of hypothetical particles over a period of 5,000 years. And researchers were surprised to learn that, although the edges of the computer-generated stream were indeed dispersed, it was hollow at its center. If this model is accurate, it may explain why Earth often experiences two periods of peak activity during a meteor shower instead of just one. From the passage, we can infer that