Solving Second-Order IVP in Matlab

I'm trying to solve the differential equation

 D2y + 0.2Dy - y + y^3 = 0.3cos(omega*t)

with the initial conditions y(0) = 0 and y'(0) = 0 for various values of omega.

I'm having a problem using dsolve to find an explicit solution - without the y^3 in the equation, things work fine, but once that's included I can't get an explicit solution out of Matlab. My code is:

 syms y t
 dsolve('D2y + 0.2*Dy - y + y^3 = 0.3*cos(0.8*t)','y(0)=0','Dy(0)=0')

Eventually I'll be plotting, but I'm okay with that. I just don't know what else to do to solve this; my teacher seemed to indicate that dsolve would work, but it isn't for me!